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  • doc/proposals/concurrency/text/concurrency.tex

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    6 Several tool can be used to solve concurrency challenges. Since many of these challenges appear with the use of mutable shared-state, some languages and libraries simply disallow mutable shared-state (Erlang~\cite{Erlang}, Haskell~\cite{Haskell}, Akka (Scala)~\cite{Akka}). In these paradigms, interaction among concurrent objects relies on message passing~\cite{Thoth,Harmony,V-Kernel} or other paradigms that closely relate to networking concepts (channels\cit for example). However, in languages that use routine calls as their core abstraction-mechanism, these approaches force a clear distinction between concurrent and non-concurrent paradigms (i.e., message passing versus routine call). This distinction in turn means that, in order to be effective, programmers need to learn two sets of designs patterns. While this distinction can be hidden away in library code, effective use of the librairy still has to take both paradigms into account.
     6Several tool can be used to solve concurrency challenges. Since many of these challenges appear with the use of mutable shared-state, some languages and libraries simply disallow mutable shared-state (Erlang~\cite{Erlang}, Haskell~\cite{Haskell}, Akka (Scala)~\cite{Akka}). In these paradigms, interaction among concurrent objects relies on message passing~\cite{Thoth,Harmony,V-Kernel} or other paradigms closely relate to networking concepts (channels\cite{CSP,Go} for example). However, in languages that use routine calls as their core abstraction-mechanism, these approaches force a clear distinction between concurrent and non-concurrent paradigms (i.e., message passing versus routine call). This distinction in turn means that, in order to be effective, programmers need to learn two sets of designs patterns. While this distinction can be hidden away in library code, effective use of the librairy still has to take both paradigms into account.
    77
    88Approaches based on shared memory are more closely related to non-concurrent paradigms since they often rely on basic constructs like routine calls and shared objects. At the lowest level, concurrent paradigms are implemented as atomic operations and locks. Many such mechanisms have been proposed, including semaphores~\cite{Dijkstra68b} and path expressions~\cite{Campbell74}. However, for productivity reasons it is desireable to have a higher-level construct be the core concurrency paradigm~\cite{HPP:Study}.
    99
    10 An approach that is worth mentionning because it is gaining in popularity is transactionnal memory~\cite{Dice10}[Check citation]. While this approach is even pursued by system languages like \CC\cit, the performance and feature set is currently too restrictive to be the main concurrency paradigm for general purpose language, which is why it was rejected as the core paradigm for concurrency in \CFA.
    11 
    12 One of the most natural, elegant, and efficient mechanisms for synchronization and communication, especially for shared memory systems, is the \emph{monitor}. Monitors were first proposed by Brinch Hansen~\cite{Hansen73} and later described and extended by C.A.R.~Hoare~\cite{Hoare74}. Many programming languages---e.g., Concurrent Pascal~\cite{ConcurrentPascal}, Mesa~\cite{Mesa}, Modula~\cite{Modula-2}, Turing~\cite{Turing:old}, Modula-3~\cite{Modula-3}, NeWS~\cite{NeWS}, Emerald~\cite{Emerald}, \uC~\cite{Buhr92a} and Java~\cite{Java}---provide monitors as explicit language constructs. In addition, operating-system kernels and device drivers have a monitor-like structure, although they often use lower-level primitives such as semaphores or locks to simulate monitors. For these reasons, this project proposes monitors as the core concurrency-construct.
     10An approach that is worth mentioning because it is gaining in popularity is transactionnal memory~\cite{Dice10}[Check citation]. While this approach is even pursued by system languages like \CC\cit, the performance and feature set is currently too restrictive to be the main concurrency paradigm for systems language, which is why it was rejected as the core paradigm for concurrency in \CFA.
     11
     12One of the most natural, elegant, and efficient mechanisms for synchronization and communication, especially for shared-memory systems, is the \emph{monitor}. Monitors were first proposed by Brinch Hansen~\cite{Hansen73} and later described and extended by C.A.R.~Hoare~\cite{Hoare74}. Many programming languages---e.g., Concurrent Pascal~\cite{ConcurrentPascal}, Mesa~\cite{Mesa}, Modula~\cite{Modula-2}, Turing~\cite{Turing:old}, Modula-3~\cite{Modula-3}, NeWS~\cite{NeWS}, Emerald~\cite{Emerald}, \uC~\cite{Buhr92a} and Java~\cite{Java}---provide monitors as explicit language constructs. In addition, operating-system kernels and device drivers have a monitor-like structure, although they often use lower-level primitives such as semaphores or locks to simulate monitors. For these reasons, this project proposes monitors as the core concurrency-construct.
    1313
    1414\section{Basics}
    15 Non-determinism requires concurrent systems to offer support for mutual-exclusion and synchronisation. Mutual-exclusion is the concept that only a fixed number of threads can access a critical section at any given time, where a critical section is a group of instructions on an associated portion of data that requires the restricted access. On the other hand, synchronization enforces relative ordering of execution and synchronization tools numerous mechanisms to establish timing relationships among threads.
     15Non-determinism requires concurrent systems to offer support for mutual-exclusion and synchronisation. Mutual-exclusion is the concept that only a fixed number of threads can access a critical section at any given time, where a critical section is a group of instructions on an associated portion of data that requires the restricted access. On the other hand, synchronization enforces relative ordering of execution and synchronization tools provide numerous mechanisms to establish timing relationships among threads.
    1616
    1717\subsection{Mutual-Exclusion}
    18 As mentionned above, mutual-exclusion is the guarantee that only a fix number of threads can enter a critical section at once. However, many solution exists for mutual exclusion which vary in terms of performance, flexibility and ease of use. Methods range from low-level locks, which are fast and flexible but require significant attention to be correct, to  higher-level mutual-exclusion methods, which sacrifice some performance in order to improve ease of use. Ease of use comes by either guaranteeing some problems cannot occur (e.g., being deadlock free) or by offering a more explicit coupling between data and corresponding critical section. For example, the \CC \code{std::atomic<T>} which offer an easy way to express mutual-exclusion on a restricted set of operations (.e.g: reading/writing large types atomically). Another challenge with low-level locks is composability. Locks are not composable because it takes careful organising for multiple locks to be used while preventing deadlocks. Easing composability is another feature higher-level mutual-exclusion mechanisms often offer.
     18As mentionned above, mutual-exclusion is the guarantee that only a fix number of threads can enter a critical section at once. However, many solutions exist for mutual exclusion, which vary in terms of performance, flexibility and ease of use. Methods range from low-level locks, which are fast and flexible but require significant attention to be correct, to  higher-level mutual-exclusion methods, which sacrifice some performance in order to improve ease of use. Ease of use comes by either guaranteeing some problems cannot occur (e.g., being deadlock free) or by offering a more explicit coupling between data and corresponding critical section. For example, the \CC \code{std::atomic<T>} offers an easy way to express mutual-exclusion on a restricted set of operations (e.g.: reading/writing large types atomically). Another challenge with low-level locks is composability. Locks have restricted composability because it takes careful organising for multiple locks to be used while preventing deadlocks. Easing composability is another feature higher-level mutual-exclusion mechanisms often offer.
    1919
    2020\subsection{Synchronization}
    21 As for mutual-exclusion, low level synchronisation primitive often offer good performance and good flexibility at the cost of ease of use. Again, higher-level mechanism often simplify usage by adding better coupling between synchronization and data, .eg., message passing, or offering simple solution to otherwise involved challenges. An example of this is barging. As mentionned above synchronization can be expressed as guaranteeing that event \textit{X} always happens before \textit{Y}. Most of the time synchronisation happens around a critical section, where threads most acquire said critical section in a certain order. However, it may also be desired to be able to guarantee that event \textit{Z} does not occur between \textit{X} and \textit{Y}. This is called barging, where event \textit{X} tries to effect event \textit{Y} but anoter thread races to grab the critical section and emits \textit{Z} before \textit{Y}. Preventing or detecting barging is an involved challenge with low-level locks, which can be made much easier by higher-level constructs.
     21As for mutual-exclusion, low-level synchronisation primitives often offer good performance and good flexibility at the cost of ease of use. Again, higher-level mechanism often simplify usage by adding better coupling between synchronization and data, e.g.: message passing, or offering simpler solution to otherwise involved challenges. As mentioned above, synchronization can be expressed as guaranteeing that event \textit{X} always happens before \textit{Y}. Most of the time, synchronisation happens within a critical section, where threads must acquire mutual-exclusion in a certain order. However, it may also be desirable to guarantee that event \textit{Z} does not occur between \textit{X} and \textit{Y}. Not satisfying this property called barging. For example, where event \textit{X} tries to effect event \textit{Y} but another thread acquires the critical section and emits \textit{Z} before \textit{Y}. The classic exmaple is the thread that finishes using a ressource and unblocks a thread waiting to use the resource, but the unblocked thread must compete again to acquire the resource. Preventing or detecting barging is an involved challenge with low-level locks, which can be made much easier by higher-level constructs. This challenge is often split into two different methods, barging avoidance and barging prevention. Algorithms that use status flags and other flag variables to detect barging threads are said to be using barging avoidance while algorithms that baton-passing locks between threads instead of releasing the locks are said to be using barging prevention.
    2222
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    2828A monitor is a set of routines that ensure mutual exclusion when accessing shared state. This concept is generally associated with Object-Oriented Languages like Java~\cite{Java} or \uC~\cite{uC++book} but does not strictly require OO semantics. The only requirements is the ability to declare a handle to a shared object and a set of routines that act on it :
    2929\begin{cfacode}
    30         typedef /*some monitor type*/ monitor;
    31         int f(monitor & m);
    32 
    33         int main() {
    34                 monitor m;  //Handle m
    35                 f(m);       //Routine using handle
    36         }
     30typedef /*some monitor type*/ monitor;
     31int f(monitor & m);
     32
     33int main() {
     34        monitor m;  //Handle m
     35        f(m);       //Routine using handle
     36}
    3737\end{cfacode}
    3838
    … …  
    4747
    4848\begin{cfacode}
    49         monitor counter_t { /*...see section $\ref{data}$...*/ };
    50 
    51         void ?{}(counter_t & nomutex this); //constructor
    52         size_t ++?(counter_t & mutex this); //increment
    53 
    54         //need for mutex is platform dependent
    55         void ?{}(size_t * this, counter_t & mutex cnt); //conversion
    56 \end{cfacode}
    57 
    58 Here, the constructor(\code{?\{\}}) uses the \code{nomutex} keyword to signify that it does not acquire the monitor mutual-exclusion when constructing. This semantics is because an object not yet constructed should never be shared and therefore does not require mutual exclusion. The prefix increment operator uses \code{mutex} to protect the incrementing process from race conditions. Finally, there is a conversion operator from \code{counter_t} to \code{size_t}. This conversion may or may not require the \code{mutex} keyword depending on whether or not reading an \code{size_t} is an atomic operation.
    59 
    60 Having both \code{mutex} and \code{nomutex} keywords is redundant based on the meaning of a routine having neither of these keywords. For example, given a routine without qualifiers \code{void foo(counter_t & this)}, then it is reasonable that it should default to the safest option \code{mutex}, whereas assuming \code{nomutex} is unsafe and may cause subtle errors. In fact, \code{nomutex} is the "normal" parameter behaviour, with the \code{nomutex} keyword effectively stating explicitly that "this routine is not special". Another alternative is to make having exactly one of these keywords mandatory, which would provide the same semantics but without the ambiguity of supporting routines neither keyword. Mandatory keywords would also have the added benefit of being self-documented but at the cost of extra typing. While there are several benefits to mandatory keywords, they do bring a few challenges. Mandatory keywords in \CFA would imply that the compiler must know without a doubt wheter or not a parameter is a monitor or not. Since \CFA relies heavily on traits as an abstraction mechanism, the distinction between a type that is a monitor and a type that looks like a monitor can become blurred. For this reason, \CFA only has the \code{mutex} keyword.
    61 
    62 
    63 The next semantic decision is to establish when \code{mutex} may be used as a type qualifier. Consider the following declarations:
    64 \begin{cfacode}
    65 int f1(monitor & mutex m);
    66 int f2(const monitor & mutex m);
    67 int f3(monitor ** mutex m);
    68 int f4(monitor * mutex m []);
    69 int f5(graph(monitor*) & mutex m);
    70 \end{cfacode}
    71 The problem is to indentify which object(s) should be acquired. Furthermore, each object needs to be acquired only once. In the case of simple routines like \code{f1} and \code{f2} it is easy to identify an exhaustive list of objects to acquire on entry. Adding indirections (\code{f3}) still allows the compiler and programmer to indentify which object is acquired. However, adding in arrays (\code{f4}) makes it much harder. Array lengths are not necessarily known in C, and even then making sure objects are only acquired once becomes none-trivial. This can be extended to absurd limits like \code{f5}, which uses a graph of monitors. To keep everyone as sane as possible~\cite{Chicken}, this projects imposes the requirement that a routine may only acquire one monitor per parameter and it must be the type of the parameter with one level of indirection (ignoring potential qualifiers). Also note that while routine \code{f3} can be supported, meaning that monitor \code{**m} is be acquired, passing an array to this routine would be type safe and yet result in undefined behavior because only the first element of the array is acquired. This is specially true for non-copyable objects like monitors, where an array of pointers is simplest way to express a group of monitors. However, this ambiguity is part of the C type-system with respects to arrays. For this reason, \code{mutex} is disallowed in the context where arrays may be passed:
    72 
    73 \begin{cfacode}
    74 int f1(monitor & mutex m);   //Okay : recommanded case
    75 int f2(monitor * mutex m);   //Okay : could be an array but probably not
    76 int f3(monitor mutex m []);  //Not Okay : Array of unkown length
    77 int f4(monitor ** mutex m);  //Not Okay : Could be an array
    78 int f5(monitor * mutex m []); //Not Okay : Array of unkown length
    79 \end{cfacode}
    80 
    81 Unlike object-oriented monitors, where calling a mutex member \emph{implicitly} acquires mutual-exclusion, \CFA uses an explicit mechanism to acquire mutual-exclusion. A consequence of this approach is that it extends naturally to multi-monitor calls.
    82 \begin{cfacode}
    83 int f(MonitorA & mutex a, MonitorB & mutex b);
    84 
    85 MonitorA a;
    86 MonitorB b;
    87 f(a,b);
    88 \end{cfacode}
    89 The capacity to acquire multiple locks before entering a critical section is called \emph{\gls{group-acquire}}. In practice, writing multi-locking routines that do not lead to deadlocks is tricky. Having language support for such a feature is therefore a significant asset for \CFA. In the case presented above, \CFA guarantees that the order of aquisition is consistent across calls to routines using the same monitors as arguments. However, since \CFA monitors use multi-acquisition locks, users can effectively force the acquiring order. For example, notice which routines use \code{mutex}/\code{nomutex} and how this affects aquiring order:
    90 \begin{cfacode}
    91         void foo(A & mutex a, B & mutex b) { //acquire a & b
    92                 ...
    93         }
    94 
    95         void bar(A & mutex a, B & /*nomutex*/ b) { //acquire a
    96                 ... foo(a, b); ... //acquire b
    97         }
    98 
    99         void baz(A & /*nomutex*/ a, B & mutex b) { //acquire b
    100                 ... foo(a, b); ... //acquire a
    101         }
    102 \end{cfacode}
    103 The multi-acquisition monitor lock allows a monitor lock to be acquired by both \code{bar} or \code{baz} and acquired again in \code{foo}. In the calls to \code{bar} and \code{baz} the monitors are acquired in opposite order.
    104 
    105 However, such use leads the lock acquiring order problem. In the example above, the user uses implicit ordering in the case of function \code{foo} but explicit ordering in the case of \code{bar} and \code{baz}. This subtle mistake means that calling these routines concurrently may lead to deadlock and is therefore undefined behavior. As shown on several occasion\cit, solving this problem requires:
    106 \begin{enumerate}
    107         \item Dynamically tracking of the monitor-call order.
    108         \item Implement rollback semantics.
    109 \end{enumerate}
    110 While the first requirement is already a significant constraint on the system, implementing a general rollback semantics in a C-like language is prohibitively complex \cit. In \CFA, users simply need to be carefull when acquiring multiple monitors at the same time.
    111 
    112 Finally, for convenience, monitors support multiple acquiring, that is acquiring a monitor while already holding it does not cause a deadlock. It simply increments an internal counter which is then used to release the monitor after the number of acquires and releases match up. This is particularly usefull when monitor routines use other monitor routines as helpers or for recursions. For example:
    113 \begin{cfacode}
    114 monitor bank {
    115         int money;
    116         log_t usr_log;
    117 };
    118 
    119 void deposit( bank & mutex b, int deposit ) {
    120         b.money += deposit;
    121         b.usr_log | "Adding" | deposit | endl;
    122 }
    123 
    124 void transfer( bank & mutex mybank, bank & mutex yourbank, int me2you) {
    125         deposit( mybank, -me2you );
    126         deposit( yourbank, me2you );
    127 }
    128 \end{cfacode}
    129 
    130 % ======================================================================
    131 % ======================================================================
    132 \subsection{Data semantics} \label{data}
    133 % ======================================================================
    134 % ======================================================================
    135 Once the call semantics are established, the next step is to establish data semantics. Indeed, until now a monitor is used simply as a generic handle but in most cases monitors contain shared data. This data should be intrinsic to the monitor declaration to prevent any accidental use of data without its appropriate protection. For example, here is a complete version of the counter showed in section \ref{call}:
    136 \begin{cfacode}
    137 monitor counter_t {
    138         int value;
    139 };
    140 
    141 void ?{}(counter_t & this) {
    142         this.cnt = 0;
    143 }
    144 
    145 int ?++(counter_t & mutex this) {
    146         return ++this.value;
    147 }
    148 
    149 //need for mutex is platform dependent here
    150 void ?{}(int * this, counter_t & mutex cnt) {
    151         *this = (int)cnt;
    152 }
    153 \end{cfacode}
    154 
     49monitor counter_t { /*...see section $\ref{data}$...*/ };
     50
     51void ?{}(counter_t & nomutex this); //constructor
     52size_t ++?(counter_t & mutex this); //increment
     53
     54//need for mutex is platform dependent
     55void ?{}(size_t * this, counter_t & mutex cnt); //conversion
     56\end{cfacode}
    15557This counter is used as follows:
    15658\begin{center}
    … …  
    16971\end{tabular}
    17072\end{center}
    171 Notice how the counter is used without any explicit synchronisation and yet supports thread-safe semantics for both reading and writting.
    172 
    173 % ======================================================================
    174 % ======================================================================
    175 \subsection{Implementation Details: Interaction with polymorphism}
    176 % ======================================================================
    177 % ======================================================================
    178 Depending on the choice of semantics for when monitor locks are acquired, interaction between monitors and \CFA's concept of polymorphism can be complex to support. However, it is shown that entry-point locking solves most of the issues.
    179 
    180 First of all, interaction between \code{otype} polymorphism and monitors is impossible since monitors do not support copying. Therefore, the main question is how to support \code{dtype} polymorphism. Since a monitor's main purpose is to ensure mutual exclusion when accessing shared data, this implies that mutual exclusion is only required for routines that do in fact access shared data. However, since \code{dtype} polymorphism always handles incomplete types (by definition), no \code{dtype} polymorphic routine can access shared data since the data requires knowledge about the type. Therefore, the only concern when combining \code{dtype} polymorphism and monitors is to protect access to routines.
    181 
    182 Before looking into complex control-flow, it is important to present the difference between the two acquiring options : callsite and entry-point locking, i.e. acquiring the monitors before making a mutex routine call or as the first operation of the mutex routine-call. For example:
     73Notice how the counter is used without any explicit synchronisation and yet supports thread-safe semantics for both reading and writting, which is similar in usage to \CC \code{atomic} template.
     74
     75Here, the constructor(\code{?\{\}}) uses the \code{nomutex} keyword to signify that it does not acquire the monitor mutual-exclusion when constructing. This semantics is because an object not yet con\-structed should never be shared and therefore does not require mutual exclusion. The prefix increment operator uses \code{mutex} to protect the incrementing process from race conditions. Finally, there is a conversion operator from \code{counter_t} to \code{size_t}. This conversion may or may not require the \code{mutex} keyword depending on whether or not reading a \code{size_t} is an atomic operation.
     76
     77For maximum usability, monitors use \gls{multi-acq} semantics, which means a single thread can acquire the same monitor multiple times without deadlock. For example, figure \ref{fig:search} uses recursion and \gls{multi-acq} to print values inside a binary tree.
     78\begin{figure}
     79\label{fig:search}
     80\begin{cfacode}
     81monitor printer { ... };
     82struct tree {
     83        tree * left, right;
     84        char * value;
     85};
     86void print(printer & mutex p, char * v);
     87
     88void print(printer & mutex p, tree * t) {
     89        print(p, t->value);
     90        print(p, t->left );
     91        print(p, t->right);
     92}
     93\end{cfacode}
     94\caption{Recursive printing algorithm using \gls{multi-acq}.}
     95\end{figure}
     96
     97Having both \code{mutex} and \code{nomutex} keywords is redundant based on the meaning of a routine having neither of these keywords. For example, given a routine without qualifiers \code{void foo(counter_t & this)}, then it is reasonable that it should default to the safest option \code{mutex}, whereas assuming \code{nomutex} is unsafe and may cause subtle errors. In fact, \code{nomutex} is the ``normal'' parameter behaviour, with the \code{nomutex} keyword effectively stating explicitly that ``this routine is not special''. Another alternative is making exactly one of these keywords mandatory, which provides the same semantics but without the ambiguity of supporting routines with neither keyword. Mandatory keywords would also have the added benefit of being self-documented but at the cost of extra typing. While there are several benefits to mandatory keywords, they do bring a few challenges. Mandatory keywords in \CFA would imply that the compiler must know without doubt whether or not a parameter is a monitor or not. Since \CFA relies heavily on traits as an abstraction mechanism, the distinction between a type that is a monitor and a type that looks like a monitor can become blurred. For this reason, \CFA only has the \code{mutex} keyword and uses no keyword to mean \code{nomutex}.
     98
     99The next semantic decision is to establish when \code{mutex} may be used as a type qualifier. Consider the following declarations:
     100\begin{cfacode}
     101int f1(monitor & mutex m);
     102int f2(const monitor & mutex m);
     103int f3(monitor ** mutex m);
     104int f4(monitor * mutex m []);
     105int f5(graph(monitor*) & mutex m);
     106\end{cfacode}
     107The problem is to indentify which object(s) should be acquired. Furthermore, each object needs to be acquired only once. In the case of simple routines like \code{f1} and \code{f2} it is easy to identify an exhaustive list of objects to acquire on entry. Adding indirections (\code{f3}) still allows the compiler and programmer to indentify which object is acquired. However, adding in arrays (\code{f4}) makes it much harder. Array lengths are not necessarily known in C, and even then making sure objects are only acquired once becomes none-trivial. This problem can be extended to absurd limits like \code{f5}, which uses a graph of monitors. To make the issue tractable, this project imposes the requirement that a routine may only acquire one monitor per parameter and it must be the type of the parameter with at most one level of indirection (ignoring potential qualifiers). Also note that while routine \code{f3} can be supported, meaning that monitor \code{**m} is be acquired, passing an array to this routine would be type safe and yet result in undefined behavior because only the first element of the array is acquired. However, this ambiguity is part of the C type-system with respects to arrays. For this reason, \code{mutex} is disallowed in the context where arrays may be passed:
     108\begin{cfacode}
     109int f1(monitor & mutex m);   //Okay : recommanded case
     110int f2(monitor * mutex m);   //Okay : could be an array but probably not
     111int f3(monitor mutex m []);  //Not Okay : Array of unkown length
     112int f4(monitor ** mutex m);  //Not Okay : Could be an array
     113int f5(monitor * mutex m []); //Not Okay : Array of unkown length
     114\end{cfacode}
     115Note that not all array functions are actually distinct in the type system. However, even if the code generation could tell the difference, the extra information is still not sufficient to extend meaningfully the monitor call semantic.
     116
     117Unlike object-oriented monitors, where calling a mutex member \emph{implicitly} acquires mutual-exclusion of the receiver object, \CFA uses an explicit mechanism to acquire mutual-exclusion. A consequence of this approach is that it extends naturally to multi-monitor calls.
     118\begin{cfacode}
     119int f(MonitorA & mutex a, MonitorB & mutex b);
     120
     121MonitorA a;
     122MonitorB b;
     123f(a,b);
     124\end{cfacode}
     125While OO monitors could be extended with a mutex qualifier for multiple-monitor calls, no example of this feature could be found. The capacity to acquire multiple locks before entering a critical section is called \emph{\gls{bulk-acq}}. In practice, writing multi-locking routines that do not lead to deadlocks is tricky. Having language support for such a feature is therefore a significant asset for \CFA. In the case presented above, \CFA guarantees that the order of aquisition is consistent across calls to different routines using the same monitors as arguments. This consistent ordering means acquiring multiple monitors in the way is safe from deadlock. However, users can still force the acquiring order. For example, notice which routines use \code{mutex}/\code{nomutex} and how this affects aquiring order:
     126\begin{cfacode}
     127void foo(A & mutex a, B & mutex b) { //acquire a & b
     128        ...
     129}
     130
     131void bar(A & mutex a, B & /*nomutex*/ b) { //acquire a
     132        ... foo(a, b); ... //acquire b
     133}
     134
     135void baz(A & /*nomutex*/ a, B & mutex b) { //acquire b
     136        ... foo(a, b); ... //acquire a
     137}
     138\end{cfacode}
     139The \gls{multi-acq} monitor lock allows a monitor lock to be acquired by both \code{bar} or \code{baz} and acquired again in \code{foo}. In the calls to \code{bar} and \code{baz} the monitors are acquired in opposite order.
     140
     141However, such use leads to the lock acquiring order problem. In the example above, the user uses implicit ordering in the case of function \code{foo} but explicit ordering in the case of \code{bar} and \code{baz}. This subtle mistake means that calling these routines concurrently may lead to deadlock and is therefore undefined behavior. As shown\cit, solving this problem requires:
     142\begin{enumerate}
     143        \item Dynamically tracking of the monitor-call order.
     144        \item Implement rollback semantics.
     145\end{enumerate}
     146While the first requirement is already a significant constraint on the system, implementing a general rollback semantics in a C-like language is prohibitively complex \cit. In \CFA, users simply need to be carefull when acquiring multiple monitors at the same time or only use \gls{bulk-acq} of all the monitors. While \CFA provides only a partial solution, many system provide no solution and the \CFA partial solution handles many useful cases.
     147
     148For example, \gls{multi-acq} and \gls{bulk-acq} can be used together in interesting ways:
     149\begin{cfacode}
     150monitor bank { ... };
     151
     152void deposit( bank & mutex b, int deposit );
     153
     154void transfer( bank & mutex mybank, bank & mutex yourbank, int me2you) {
     155        deposit( mybank, -me2you );
     156        deposit( yourbank, me2you );
     157}
     158\end{cfacode}
     159This example shows a trivial solution to the bank-account transfer-problem\cit. Without \gls{multi-acq} and \gls{bulk-acq}, the solution to this problem is much more involved and requires carefull engineering.
     160
     161\subsection{\code{mutex} statement} \label{mutex-stmt}
     162
     163The call semantics discussed aboved have one software engineering issue, only a named routine can acquire the mutual-exclusion of a set of monitor. \CFA offers the \code{mutex} statement to workaround the need for unnecessary names, avoiding a major software engineering problem\cit. Listing \ref{lst:mutex-stmt} shows an example of the \code{mutex} statement, which introduces a new scope in which the mutual-exclusion of a set of monitor is acquired. Beyond naming, the \code{mutex} statement has no semantic difference from a routine call with \code{mutex} parameters.
     164
     165\begin{figure}
    183166\begin{center}
    184 \setlength\tabcolsep{1.5pt}
    185 \begin{tabular}{|c|c|c|}
    186 Code & \gls{callsite-locking} & \gls{entry-point-locking} \\
    187 \CFA & pseudo-code & pseudo-code \\
     167\begin{tabular}{|c|c|}
     168function call & \code{mutex} statement \\
    188169\hline
    189170\begin{cfacode}[tabsize=3]
    190 void foo(monitor& mutex a){
    191 
    192 
    193 
    194         //Do Work
    195         //...
    196 
    197 }
    198 
    199 void main() {
    200         monitor a;
    201 
    202 
    203 
    204         foo(a);
    205 
    206 }
    207 \end{cfacode} & \begin{pseudo}[tabsize=3]
    208 foo(& a) {
    209 
    210 
    211 
    212         //Do Work
    213         //...
    214 
    215 }
    216 
    217 main() {
    218         monitor a;
    219         //calling routine
    220         //handles concurrency
    221         acquire(a);
    222         foo(a);
    223         release(a);
    224 }
    225 \end{pseudo} & \begin{pseudo}[tabsize=3]
    226 foo(& a) {
    227         //called routine
    228         //handles concurrency
    229         acquire(a);
    230         //Do Work
    231         //...
    232         release(a);
    233 }
    234 
    235 main() {
    236         monitor a;
    237 
    238 
    239 
    240         foo(a);
    241 
    242 }
    243 \end{pseudo}
     171monitor M {};
     172void foo( M & mutex m ) {
     173        //critical section
     174}
     175
     176void bar( M & m ) {
     177        foo( m );
     178}
     179\end{cfacode}&\begin{cfacode}[tabsize=3]
     180monitor M {};
     181void bar( M & m ) {
     182        mutex(m) {
     183                //critical section
     184        }
     185}
     186
     187
     188\end{cfacode}
    244189\end{tabular}
    245190\end{center}
    246 
    247 \Gls{callsite-locking} is inefficient, since any \code{dtype} routine may have to obtain some lock before calling a routine, depending on whether or not the type passed is a monitor. However, with \gls{entry-point-locking} calling a monitor routine becomes exactly the same as calling it from anywhere else.
    248 
    249 Note the \code{mutex} keyword relies on the resolver, which means that in cases where a generic monitor routine is actually desired, writing a mutex routine is possible with the proper trait. This is possible because monitors are designed in terms a trait. For example:
    250 \begin{cfacode}
    251 //Incorrect
    252 //T is not a monitor
    253 forall(dtype T)
    254 void foo(T * mutex t);
    255 
    256 //Correct
    257 //this function only works on monitors
    258 //(any monitor)
    259 forall(dtype T | is_monitor(T))
    260 void bar(T * mutex t));
    261 \end{cfacode}
    262 
    263 
    264 % ======================================================================
    265 % ======================================================================
    266 \section{Internal scheduling} \label{insched}
    267 % ======================================================================
    268 % ======================================================================
    269 In addition to mutual exclusion, the monitors at the core of \CFA's concurrency can also be used to achieve synchronisation. With monitors, this is generally achieved with internal or external scheduling as in\cit. Since internal scheduling of single monitors is mostly a solved problem, this proposal concentraits on extending internal scheduling to multiple monitors at once. Indeed, like the \gls{group-acquire} semantics, internal scheduling extends to multiple monitors at once in a way that is natural to the user but requires additional complexity on the implementation side.
     191\caption{Regular call semantics vs. \code{mutex} statement}
     192\label{lst:mutex-stmt}
     193\end{figure}
     194
     195% ======================================================================
     196% ======================================================================
     197\subsection{Data semantics} \label{data}
     198% ======================================================================
     199% ======================================================================
     200Once the call semantics are established, the next step is to establish data semantics. Indeed, until now a monitor is used simply as a generic handle but in most cases monitors contain shared data. This data should be intrinsic to the monitor declaration to prevent any accidental use of data without its appropriate protection. For example, here is a complete version of the counter showed in section \ref{call}:
     201\begin{cfacode}
     202monitor counter_t {
     203        int value;
     204};
     205
     206void ?{}(counter_t & this) {
     207        this.cnt = 0;
     208}
     209
     210int ?++(counter_t & mutex this) {
     211        return ++this.value;
     212}
     213
     214//need for mutex is platform dependent here
     215void ?{}(int * this, counter_t & mutex cnt) {
     216        *this = (int)cnt;
     217}
     218\end{cfacode}
     219
     220Like threads and coroutines, monitors are defined in terms of traits with some additional language support in the form of the \code{monitor} keyword. The monitor trait is :
     221\begin{cfacode}
     222trait is_monitor(dtype T) {
     223        monitor_desc * get_monitor( T & );
     224        void ^?{}( T & mutex );
     225};
     226\end{cfacode}
     227Note that the destructor of a monitor must be a \code{mutex} routine. This requirement ensures that the destructor has mutual-exclusion. As with any object, any call to a monitor, using \code{mutex} or otherwise, is Undefined Behaviour after the destructor has run.
     228
     229% ======================================================================
     230% ======================================================================
     231\section{Internal scheduling} \label{intsched}
     232% ======================================================================
     233% ======================================================================
     234In addition to mutual exclusion, the monitors at the core of \CFA's concurrency can also be used to achieve synchronisation. With monitors, this capability is generally achieved with internal or external scheduling as in\cit. Since internal scheduling within a single monitor is mostly a solved problem, this thesis concentrates on extending internal scheduling to multiple monitors. Indeed, like the \gls{bulk-acq} semantics, internal scheduling extends to multiple monitors in a way that is natural to the user but requires additional complexity on the implementation side.
    270235
    271236First, here is a simple example of such a technique:
    272237
    273238\begin{cfacode}
    274         monitor A {
    275                 condition e;
    276         }
    277 
    278         void foo(A & mutex a) {
    279                 ...
    280                 // Wait for cooperation from bar()
    281                 wait(a.e);
    282                 ...
    283         }
    284 
    285         void bar(A & mutex a) {
    286                 // Provide cooperation for foo()
    287                 ...
    288                 // Unblock foo at scope exit
    289                 signal(a.e);
    290         }
    291 \end{cfacode}
    292 
    293 There are two details to note here. First, there \code{signal} is a delayed operation, it only unblocks the waiting thread when it reaches the end of the critical section. This is needed to respect mutual-exclusion. Second, in \CFA, \code{condition} have no particular need to be stored inside a monitor, beyond any software engineering reasons. Here routine \code{foo} waits for the \code{signal} from \code{bar} before making further progress, effectively ensuring a basic ordering.
    294 
    295 An important aspect to take into account here is that \CFA does not allow barging, which means that once function \code{bar} releases the monitor, foo is guaranteed to resume immediately after (unless some other thread waited on the same condition). This guarantees offers the benefit of not having to loop arount waits in order to guarantee that a condition is still met. The main reason \CFA offers this guarantee is that users can easily introduce barging if it becomes a necessity but adding barging prevention or barging avoidance is more involved without language support. Supporting barging prevention as well as extending internal scheduling to multiple monitors is the main source of complexity in the design of \CFA concurrency.
     239monitor A {
     240        condition e;
     241}
     242
     243void foo(A & mutex a) {
     244        ...
     245        //Wait for cooperation from bar()
     246        wait(a.e);
     247        ...
     248}
     249
     250void bar(A & mutex a) {
     251        //Provide cooperation for foo()
     252        ...
     253        //Unblock foo
     254        signal(a.e);
     255}
     256\end{cfacode}
     257
     258There are two details to note here. First, the \code{signal} is a delayed operation, it only unblocks the waiting thread when it reaches the end of the critical section. This semantic is needed to respect mutual-exclusion. The alternative is to return immediately after the call to \code{signal}, which is significantly more restrictive. Second, in \CFA, while it is common to store a \code{condition} as a field of the monitor, a \code{condition} variable can be stored/created independently of a monitor. Here routine \code{foo} waits for the \code{signal} from \code{bar} before making further progress, effectively ensuring a basic ordering.
     259
     260An important aspect of the implementation is that \CFA does not allow barging, which means that once function \code{bar} releases the monitor, \code{foo} is guaranteed to resume immediately after (unless some other thread waited on the same condition). This guarantees offers the benefit of not having to loop arount waits in order to guarantee that a condition is still met. The main reason \CFA offers this guarantee is that users can easily introduce barging if it becomes a necessity but adding barging prevention or barging avoidance is more involved without language support. Supporting barging prevention as well as extending internal scheduling to multiple monitors is the main source of complexity in the design of \CFA concurrency.
    296261
    297262% ======================================================================
    … …  
    300265% ======================================================================
    301266% ======================================================================
    302 It is easier to understand the problem of multi-monitor scheduling using a series of pseudo-code. Note that for simplicity in the following snippets of pseudo-code, waiting and signalling is done using an implicit condition variable, like Java built-in monitors.
     267It is easier to understand the problem of multi-monitor scheduling using a series of pseudo-code. Note that for simplicity in the following snippets of pseudo-code, waiting and signalling is done using an implicit condition variable, like Java built-in monitors. Indeed, \code{wait} statements always use the implicit condition as paremeter and explicitly names the monitors (A and B) associated with the condition. Note that in \CFA, condition variables are tied to a set of monitors on first use (called branding) which means that using internal scheduling with distinct sets of monitors requires one condition variable per set of monitors.
    303268
    304269\begin{multicols}{2}
    … …  
    319284\end{pseudo}
    320285\end{multicols}
    321 The example shows the simple case of having two threads (one for each column) and a single monitor A. One thread acquires before waiting (atomically blocking and releasing A) and the other acquires before signalling. There is an important thing to note here, both \code{wait} and \code{signal} must be called with the proper monitor(s) already acquired. This restriction is hidden on the user side in \uC, as it is a logical requirement for barging prevention.
    322 
    323 A direct extension of the previous example is the \gls{group-acquire} version:
     286The example shows the simple case of having two threads (one for each column) and a single monitor A. One thread acquires before waiting (atomically blocking and releasing A) and the other acquires before signalling. It is important to note here that both \code{wait} and \code{signal} must be called with the proper monitor(s) already acquired. This semantic is a logical requirement for barging prevention.
     287
     288A direct extension of the previous example is a \gls{bulk-acq} version:
    324289
    325290\begin{multicols}{2}
    … …  
    338303\end{pseudo}
    339304\end{multicols}
    340 This version uses \gls{group-acquire} (denoted using the \& symbol), but the presence of multiple monitors does not add a particularly new meaning. Synchronization happens between the two threads in exactly the same way and order. The only difference is that mutual exclusion covers more monitors. On the implementation side, handling multiple monitors does add a degree of complexity as the next few examples demonstrate.
    341 
    342 While deadlock issues can occur when nesting monitors, these issues are only a symptom of the fact that locks, and by extension monitors, are not perfectly composable. However, for monitors as for locks, it is possible to write a program using nesting without encountering any problems if nested is done correctly. For example, the next pseudo-code snippet acquires monitors A then B before waiting while only acquiring B when signalling, effectively avoiding the nested monitor problem.
    343 
     305This version uses \gls{bulk-acq} (denoted using the {\sf\&} symbol), but the presence of multiple monitors does not add a particularly new meaning. Synchronization happens between the two threads in exactly the same way and order. The only difference is that mutual exclusion covers more monitors. On the implementation side, handling multiple monitors does add a degree of complexity as the next few examples demonstrate.
     306
     307While deadlock issues can occur when nesting monitors, these issues are only a symptom of the fact that locks, and by extension monitors, are not perfectly composable. For monitors, a well known deadlock problem is the Nested Monitor Problem\cit, which occurs when a \code{wait} is made by a thread that holds more than one monitor. For example, the following pseudo-code runs into the nested-monitor problem :
    344308\begin{multicols}{2}
    345309\begin{pseudo}
    … …  
    354318
    355319\begin{pseudo}
     320acquire A
     321        acquire B
     322                signal B
     323        release B
     324release A
     325\end{pseudo}
     326\end{multicols}
     327
     328The \code{wait} only releases monitor \code{B} so the signalling thread cannot acquire monitor \code{A} to get to the \code{signal}. Attempting release of all acquired monitors at the \code{wait} results in another set of problems such as releasing monitor \code{C}, which has nothing to do with the \code{signal}.
     329
     330However, for monitors as for locks, it is possible to write a program using nesting without encountering any problems if nesting is done correctly. For example, the next pseudo-code snippet acquires monitors {\sf A} then {\sf B} before waiting, while only acquiring {\sf B} when signalling, effectively avoiding the nested monitor problem.
     331
     332\begin{multicols}{2}
     333\begin{pseudo}
     334acquire A
     335        acquire B
     336                wait B
     337        release B
     338release A
     339\end{pseudo}
     340
     341\columnbreak
     342
     343\begin{pseudo}
    356344
    357345acquire B
    … …  
    362350\end{multicols}
    363351
    364 The next example is where \gls{group-acquire} adds a significant layer of complexity to the internal signalling semantics.
    365 
     352% ======================================================================
     353% ======================================================================
     354\subsection{Internal Scheduling - in depth}
     355% ======================================================================
     356% ======================================================================
     357
     358A larger example is presented to show complex issuesfor \gls{bulk-acq} and all the implementation options are analyzed. Listing \ref{lst:int-bulk-pseudo} shows an example where \gls{bulk-acq} adds a significant layer of complexity to the internal signalling semantics, and listing \ref{lst:int-bulk-cfa} shows the corresponding \CFA code which implements the pseudo-code in listing \ref{lst:int-bulk-pseudo}. For the purpose of translating the given pseudo-code into \CFA-code any method of introducing monitor into context, other than a \code{mutex} parameter, is acceptable, e.g., global variables, pointer parameters or using locals with the \code{mutex}-statement.
     359
     360\begin{figure}[!b]
    366361\begin{multicols}{2}
    367362Waiting thread
    368363\begin{pseudo}[numbers=left]
    369364acquire A
    370         // Code Section 1
     365        //Code Section 1
    371366        acquire A & B
    372                 // Code Section 2
     367                //Code Section 2
    373368                wait A & B
    374                 // Code Section 3
     369                //Code Section 3
    375370        release A & B
    376         // Code Section 4
     371        //Code Section 4
    377372release A
    378373\end{pseudo}
    … …  
    383378\begin{pseudo}[numbers=left, firstnumber=10]
    384379acquire A
    385         // Code Section 5
     380        //Code Section 5
    386381        acquire A & B
    387                 // Code Section 6
     382                //Code Section 6
    388383                signal A & B
    389                 // Code Section 7
     384                //Code Section 7
    390385        release A & B
    391         // Code Section 8
     386        //Code Section 8
    392387release A
    393388\end{pseudo}
    394389\end{multicols}
     390\caption{Internal scheduling with \gls{bulk-acq}}
     391\label{lst:int-bulk-pseudo}
     392\end{figure}
     393
     394\begin{figure}[!b]
    395395\begin{center}
    396 Listing 1
     396\begin{cfacode}[xleftmargin=.4\textwidth]
     397monitor A a;
     398monitor B b;
     399condition c;
     400\end{cfacode}
    397401\end{center}
    398 
    399 It is particularly important to pay attention to code sections 8 and 4, which are where the existing semantics of internal scheduling need to be extended for multiple monitors. The root of the problem is that \gls{group-acquire} is used in a context where one of the monitors is already acquired and is why it is important to define the behaviour of the previous pseudo-code. When the signaller thread reaches the location where it should "release A \& B" (line 16), it must actually transfer ownership of monitor B to the waiting thread. This ownership trasnfer is required in order to prevent barging. Since the signalling thread still needs the monitor A, simply waking up the waiting thread is not an option because it would violate mutual exclusion. There are three options:
     402\begin{multicols}{2}
     403Waiting thread
     404\begin{cfacode}
     405mutex(a) {
     406        //Code Section 1
     407        mutex(a, b) {
     408                //Code Section 2
     409                wait(c);
     410                //Code Section 3
     411        }
     412        //Code Section 4
     413}
     414\end{cfacode}
     415
     416\columnbreak
     417
     418Signalling thread
     419\begin{cfacode}
     420mutex(a) {
     421        //Code Section 5
     422        mutex(a, b) {
     423                //Code Section 6
     424                signal(c);
     425                //Code Section 7
     426        }
     427        //Code Section 8
     428}
     429\end{cfacode}
     430\end{multicols}
     431\caption{Equivalent \CFA code for listing \ref{lst:int-bulk-pseudo}}
     432\label{lst:int-bulk-cfa}
     433\end{figure}
     434
     435The complexity begins at code sections 4 and 8, which are where the existing semantics of internal scheduling need to be extended for multiple monitors. The root of the problem is that \gls{bulk-acq} is used in a context where one of the monitors is already acquired and is why it is important to define the behaviour of the previous pseudo-code. When the signaller thread reaches the location where it should ``release \code{A & B}'' (line 16), it must actually transfer ownership of monitor \code{B} to the waiting thread. This ownership trasnfer is required in order to prevent barging. Since the signalling thread still needs monitor \code{A}, simply waking up the waiting thread is not an option because it violates mutual exclusion. There are three options.
    400436
    401437\subsubsection{Delaying signals}
    402 The first more obvious solution to solve the problem of multi-monitor scheduling is to keep ownership of all locks until the last lock is ready to be transferred. It can be argued that that moment is the correct time to transfer ownership when the last lock is no longer needed because this semantics fits most closely to the behaviour of single monitor scheduling. This solution has the main benefit of transferring ownership of groups of monitors, which simplifies the semantics from mutiple objects to a single group of object, effectively making the existing single monitor semantic viable by simply changing monitors to monitor collections.
     438The obvious solution to solve the problem of multi-monitor scheduling is to keep ownership of all locks until the last lock is ready to be transferred. It can be argued that that moment is when the last lock is no longer needed because this semantics fits most closely to the behaviour of single-monitor scheduling. This solution has the main benefit of transferring ownership of groups of monitors, which simplifies the semantics from mutiple objects to a single group of objects, effectively making the existing single-monitor semantic viable by simply changing monitors to monitor groups.
    403439\begin{multicols}{2}
    404440Waiter
    … …  
    424460\end{pseudo}
    425461\end{multicols}
    426 However, this solution can become much more complicated depending on what is executed while secretly holding B (at line 10). Indeed, nothing prevents a user from signalling monitor A on a different condition variable:
    427 \newpage
    428 \begin{multicols}{2}
    429 Thread 1
     462However, this solution can become much more complicated depending on what is executed while secretly holding B (at line 10). Indeed, nothing prevents signalling monitor A on a different condition variable:
     463\begin{figure}
     464\begin{multicols}{3}
     465Thread $\alpha$
    430466\begin{pseudo}[numbers=left, firstnumber=1]
    431467acquire A
    … …  
    436472\end{pseudo}
    437473
    438 Thread 2
    439 \begin{pseudo}[numbers=left, firstnumber=6]
    440 acquire A
    441         wait A
    442 release A
    443 \end{pseudo}
    444 
    445474\columnbreak
    446475
    447 Thread 3
    448 \begin{pseudo}[numbers=left, firstnumber=10]
     476Thread $\gamma$
     477\begin{pseudo}[numbers=left, firstnumber=1]
    449478acquire A
    450479        acquire A & B
    451480                signal A & B
    452481        release A & B
    453         //Secretly keep B here
    454482        signal A
    455483release A
    456 //Wakeup thread 1 or 2?
    457 //Who wakes up the other thread?
    458 \end{pseudo}
     484\end{pseudo}
     485
     486\columnbreak
     487
     488Thread $\beta$
     489\begin{pseudo}[numbers=left, firstnumber=1]
     490acquire A
     491        wait A
     492release A
     493\end{pseudo}
     494
    459495\end{multicols}
     496\caption{Dependency graph}
     497\label{lst:dependency}
     498\end{figure}
    460499
    461500The goal in this solution is to avoid the need to transfer ownership of a subset of the condition monitors. However, this goal is unreacheable in the previous example. Depending on the order of signals (line 12 and 15) two cases can happen.
    … …  
    467506Note that ordering is not determined by a race condition but by whether signalled threads are enqueued in FIFO or FILO order. However, regardless of the answer, users can move line 15 before line 11 and get the reverse effect.
    468507
    469 In both cases, the threads need to be able to distinguish on a per monitor basis which ones need to be released and which ones need to be transferred. Which means monitors cannot be handled as a single homogenous group.
     508In both cases, the threads need to be able to distinguish, on a per monitor basis, which ones need to be released and which ones need to be transferred, which means monitors cannot be handled as a single homogenous group and therefore effectively precludes this approach.
    470509
    471510\subsubsection{Dependency graphs}
    472 In the Listing 1 pseudo-code, there is a solution which statisfies both barging prevention and mutual exclusion. If ownership of both monitors is transferred to the waiter when the signaller releases A and then the waiter transfers back ownership of A when it releases it then the problem is solved. Dynamically finding the correct order is therefore the second possible solution. The problem it encounters is that it effectively boils down to resolving a dependency graph of ownership requirements. Here even the simplest of code snippets requires two transfers and it seems to increase in a manner closer to polynomial. For example, the following code, which is just a direct extension to three monitors, requires at least three ownership transfer and has multiple solutions:
     511In the listing \ref{lst:int-bulk-pseudo} pseudo-code, there is a solution which statisfies both barging prevention and mutual exclusion. If ownership of both monitors is transferred to the waiter when the signaller releases \code{A & B} and then the waiter transfers back ownership of \code{A} when it releases it, then the problem is solved (\code{B} is no longer in use at this point). Dynamically finding the correct order is therefore the second possible solution. The problem it encounters is that it effectively boils down to resolving a dependency graph of ownership requirements. Here even the simplest of code snippets requires two transfers and it seems to increase in a manner closer to polynomial. For example, the following code, which is just a direct extension to three monitors, requires at least three ownership transfer and has multiple solutions:
    473512
    474513\begin{multicols}{2}
    … …  
    495534\end{pseudo}
    496535\end{multicols}
    497 Resolving dependency graph being a complex and expensive endeavour, this solution is not the preffered one.
     536
     537\begin{figure}
     538\begin{center}
     539\input{dependency}
     540\end{center}
     541\caption{Dependency graph of the statements in listing \ref{lst:dependency}}
     542\label{fig:dependency}
     543\end{figure}
     544
     545Listing \ref{lst:dependency} is the three thread example rewritten for dependency graphs. Figure \ref{fig:dependency} shows the corresponding dependency graph that results, where every node is a statement of one of the three threads, and the arrows the dependency of that statement (e.g., $\alpha1$ must happen before $\alpha2$). The extra challenge is that this dependency graph is effectively post-mortem, but the runtime system needs to be able to build and solve these graphs as the dependency unfolds. Resolving dependency graph being a complex and expensive endeavour, this solution is not the preffered one.
    498546
    499547\subsubsection{Partial signalling} \label{partial-sig}
    500 Finally, the solution that is chosen for \CFA is to use partial signalling. Consider the following case:
    501 
    502 \begin{multicols}{2}
    503 \begin{pseudo}[numbers=left]
    504 acquire A
    505         acquire A & B
    506                 wait A & B
    507         release A & B
    508 release A
    509 \end{pseudo}
    510 
    511 \columnbreak
    512 
    513 \begin{pseudo}[numbers=left, firstnumber=6]
    514 acquire A
    515         acquire A & B
    516                 signal A & B
    517         release A & B
    518         // ... More code
    519 release A
    520 \end{pseudo}
    521 \end{multicols}
    522 The partial signalling solution transfers ownership of monitor B at lines 10 but does not wake the waiting thread since it is still using monitor A. Only when it reaches line 11 does it actually wakeup the waiting thread. This solution has the benefit that complexity is encapsulated into only two actions, passing monitors to the next owner when they should be release and conditionnaly waking threads if all conditions are met. Contrary to the other solutions, this solution quickly hits an upper bound on complexity of implementation.
     548Finally, the solution that is chosen for \CFA is to use partial signalling. Again using listing \ref{lst:int-bulk-pseudo}, the partial signalling solution transfers ownership of monitor B at lines 10 but does not wake the waiting thread since it is still using monitor A. Only when it reaches line 11 does it actually wakeup the waiting thread. This solution has the benefit that complexity is encapsulated into only two actions, passing monitors to the next owner when they should be release and conditionally waking threads if all conditions are met. This solution has a much simpler implementation than a dependency graph solving algorithm which is why it was chosen. Furthermore, after being fully implemented, this solution does not appear to have any downsides worth mentionning.
    523549
    524550% ======================================================================
    … …  
    527553% ======================================================================
    528554% ======================================================================
    529 An important note is that, until now, signalling a monitor was a delayed operation. The ownership of the monitor is transferred only when the monitor would have otherwise been released, not at the point of the \code{signal} statement. However, in some cases, it may be more convenient for users to immediately transfer ownership to the thread that is waiting for cooperation, which is achieved using the \code{signal_block} routine\footnote{name to be discussed}.
    530 
    531 For example here is an example highlighting the difference in behaviour:
    532 \begin{center}
     555\begin{figure}
    533556\begin{tabular}{|c|c|}
    534557\code{signal} & \code{signal_block} \\
    535558\hline
    536 \begin{cfacode}
    537 monitor M { int val; };
    538 
    539 void foo(M & mutex m ) {
    540         m.val++;
    541         sout| "Foo:" | m.val |endl;
    542 
    543         wait( c );
    544 
    545         m.val++;
    546         sout| "Foo:" | m.val |endl;
    547 }
    548 
    549 void bar(M & mutex m ) {
    550         m.val++;
    551         sout| "Bar:" | m.val |endl;
    552 
    553         signal( c );
    554 
    555         m.val++;
    556         sout| "Bar:" | m.val |endl;
    557 }
    558 \end{cfacode}&\begin{cfacode}
    559 monitor M { int val; };
    560 
    561 void foo(M & mutex m ) {
    562         m.val++;
    563         sout| "Foo:" | m.val |endl;
    564 
    565         wait( c );
    566 
    567         m.val++;
    568         sout| "Foo:" | m.val |endl;
    569 }
    570 
    571 void bar(M & mutex m ) {
    572         m.val++;
    573         sout| "Bar:" | m.val |endl;
    574 
    575         signal_block( c );
    576 
    577         m.val++;
    578         sout| "Bar:" | m.val |endl;
     559\begin{cfacode}[tabsize=3]
     560monitor DatingService
     561{
     562        //compatibility codes
     563        enum{ CCodes = 20 };
     564
     565        int girlPhoneNo
     566        int boyPhoneNo;
     567};
     568
     569condition girls[CCodes];
     570condition boys [CCodes];
     571condition exchange;
     572
     573int girl(int phoneNo, int ccode)
     574{
     575        //no compatible boy ?
     576        if(empty(boys[ccode]))
     577        {
     578                //wait for boy
     579                wait(girls[ccode]);
     580
     581                //make phone number available
     582                girlPhoneNo = phoneNo;
     583
     584                //wake boy from chair
     585                signal(exchange);
     586        }
     587        else
     588        {
     589                //make phone number available
     590                girlPhoneNo = phoneNo;
     591
     592                //wake boy
     593                signal(boys[ccode]);
     594
     595                //sit in chair
     596                wait(exchange);
     597        }
     598        return boyPhoneNo;
     599}
     600
     601int boy(int phoneNo, int ccode)
     602{
     603        //same as above
     604        //with boy/girl interchanged
     605}
     606\end{cfacode}&\begin{cfacode}[tabsize=3]
     607monitor DatingService
     608{
     609        //compatibility codes
     610        enum{ CCodes = 20 };
     611
     612        int girlPhoneNo;
     613        int boyPhoneNo;
     614};
     615
     616condition girls[CCodes];
     617condition boys [CCodes];
     618//exchange is not needed
     619
     620int girl(int phoneNo, int ccode)
     621{
     622        //no compatible boy ?
     623        if(empty(boys[ccode]))
     624        {
     625                //wait for boy
     626                wait(girls[ccode]);
     627
     628                //make phone number available
     629                girlPhoneNo = phoneNo;
     630
     631                //wake boy from chair
     632                signal(exchange);
     633        }
     634        else
     635        {
     636                //make phone number available
     637                girlPhoneNo = phoneNo;
     638
     639                //wake boy
     640                signal_block(boys[ccode]);
     641
     642                //second handshake unnecessary
     643
     644        }
     645        return boyPhoneNo;
     646}
     647
     648int boy(int phoneNo, int ccode)
     649{
     650        //same as above
     651        //with boy/girl interchanged
    579652}
    580653\end{cfacode}
    581654\end{tabular}
    582 \end{center}
    583 Assuming that \code{val} is initialized at 0, that each routine are called from seperate thread and that \code{foo} is always called first. The previous code would yield the following output:
    584 
     655\caption{Dating service example using \code{signal} and \code{signal_block}. }
     656\label{lst:datingservice}
     657\end{figure}
     658An important note is that, until now, signalling a monitor was a delayed operation. The ownership of the monitor is transferred only when the monitor would have otherwise been released, not at the point of the \code{signal} statement. However, in some cases, it may be more convenient for users to immediately transfer ownership to the thread that is waiting for cooperation, which is achieved using the \code{signal_block} routine\footnote{name to be discussed}.
     659
     660The example in listing \ref{lst:datingservice} highlights the difference in behaviour. As mentioned, \code{signal} only transfers ownership once the current critical section exits, this behaviour requires additional synchronisation when a two-way handshake is needed. To avoid this extraneous synchronisation, the \code{condition} type offers the \code{signal_block} routine, which handles the two-way handshake as shown in the example. This removes the need for a second condition variables and simplifies programming. Like every other monitor semantic, \code{signal_block} uses barging prevention, which means mutual-exclusion is baton-passed both on the frond-end and the back-end of the call to \code{signal_block}, meaning no other thread can acquire the monitor neither before nor after the call.
     661
     662% ======================================================================
     663% ======================================================================
     664\section{External scheduling} \label{extsched}
     665% ======================================================================
     666% ======================================================================
     667An alternative to internal scheduling is external scheduling, e.g., in \uC.
    585668\begin{center}
    586 \begin{tabular}{|c|c|}
    587 \code{signal} & \code{signal_block} \\
     669\begin{tabular}{|c|c|c|}
     670Internal Scheduling & External Scheduling & Go\\
    588671\hline
    589 \begin{pseudo}
    590 Foo: 0
    591 Bar: 1
    592 Bar: 2
    593 Foo: 3
    594 \end{pseudo}&\begin{pseudo}
    595 Foo: 0
    596 Bar: 1
    597 Foo: 2
    598 Bar: 3
    599 \end{pseudo}
    600 \end{tabular}
    601 \end{center}
    602 
    603 As mentionned, \code{signal} only transfers ownership once the current critical section exits, resulting in the second "Bar" line to be printed before the second "Foo" line. On the other hand, \code{signal_block} immediately transfers ownership to \code{bar}, causing an inversion of output. Obviously this means that \code{signal_block} is a blocking call, which will only be resumed once the signalled function exits the critical section.
    604 
    605 % ======================================================================
    606 % ======================================================================
    607 \subsection{Internal scheduling: Implementation} \label{inschedimpl}
    608 % ======================================================================
    609 % ======================================================================
    610 There are several challenges specific to \CFA when implementing internal scheduling. These challenges are direct results of \gls{group-acquire} and loose object definitions. These two constraints are to root cause of most design decisions in the implementation of internal scheduling. Furthermore, to avoid the head-aches of dynamically allocating memory in a concurrent environment, the internal-scheduling design is entirely free of mallocs and other dynamic memory allocation scheme. This is to avoid the chicken and egg problem of having a memory allocator that relies on the threading system and a threading system that relies on the runtime. This extra goal, means that memory management is a constant concern in the design of the system.
    611 
    612 The main memory concern for concurrency is queues. All blocking operations are made by parking threads onto queues. These queues need to be intrinsic\cit to avoid the need memory allocation. This entails that all the fields needed to keep track of all needed information. Since internal scheduling can use an unbound amount of memory (depending on \gls{group-acquire}) statically defining information information in the intrusive fields of threads is insufficient. The only variable sized container that does not require memory allocation is the callstack, which is heavily used in the implementation of internal scheduling. Particularly the GCC extension variable length arrays which is used extensively.
    613 
    614 Since stack allocation is based around scope, the first step of the implementation is to identify the scopes that are available to store the information, and which of these can have a variable length. In the case of external scheduling, the threads and the condition both allow a fixed amount of memory to be stored, while mutex-routines and the actual blocking call allow for an unbound amount (though adding too much to the mutex routine stack size can become expansive faster).
    615 
    616 The following figure is the traditionnal illustration of a monitor :
    617 
    618 \begin{center}
    619 {\resizebox{0.4\textwidth}{!}{\input{monitor}}}
    620 \end{center}
    621 
    622 For \CFA, the previous picture does not have support for blocking multiple monitors on a single condition. To support \gls{group-acquire} two changes to this picture are required. First, it doesn't make sense to tie the condition to a single monitor since blocking two monitors as one would require arbitrarily picking a monitor to hold the condition. Secondly, the object waiting on the conditions and AS-stack cannot simply contain the waiting thread since a single thread can potentially wait on multiple monitors. As mentionned in section \ref{inschedimpl}, the handling in multiple monitors is done by partially passing, which entails that each concerned monitor needs to have a node object. However, for waiting on the condition, since all threads need to wait together, a single object needs to be queued in the condition. Moving out the condition and updating the node types yields :
    623 
    624 \begin{center}
    625 {\resizebox{0.8\textwidth}{!}{\input{int_monitor}}}
    626 \end{center}
    627 
    628 \newpage
    629 
    630 This picture and the proper entry and leave algorithms is the fundamental implementation of internal scheduling.
    631 
    632 \begin{multicols}{2}
    633 Entry
    634 \begin{pseudo}[numbers=left]
    635 if monitor is free
    636         enter
    637 elif I already own the monitor
    638         continue
    639 else
    640         block
    641 increment recursion
    642 
    643 \end{pseudo}
    644 \columnbreak
    645 Exit
    646 \begin{pseudo}[numbers=left, firstnumber=8]
    647 decrement recursion
    648 if recursion == 0
    649         if signal_stack not empty
    650                 set_owner to thread
    651                 if all monitors ready
    652                         wake-up thread
    653 
    654         if entry queue not empty
    655                 wake-up thread
    656 \end{pseudo}
    657 \end{multicols}
    658 
    659 Some important things to notice about the exit routine. The solution discussed in \ref{inschedimpl} can be seen on line 11 of the previous pseudo code. Basically, the solution boils down to having a seperate data structure for the condition queue and the AS-stack, and unconditionally transferring ownership of the monitors but only unblocking the thread when the last monitor has trasnferred ownership. This solution is safe as well as preventing any potential barging.
    660 
    661 % ======================================================================
    662 % ======================================================================
    663 \section{External scheduling} \label{extsched}
    664 % ======================================================================
    665 % ======================================================================
    666 An alternative to internal scheduling is to use external scheduling.
    667 \begin{center}
    668 \begin{tabular}{|c|c|}
    669 Internal Scheduling & External Scheduling \\
    670 \hline
    671 \begin{ucppcode}
     672\begin{ucppcode}[tabsize=3]
    672673_Monitor Semaphore {
    673674        condition c;
    … …  
    675676public:
    676677        void P() {
    677                 if(inUse) wait(c);
     678                if(inUse)
     679                        wait(c);
    678680                inUse = true;
    679681        }
    … …  
    683685        }
    684686}
    685 \end{ucppcode}&\begin{ucppcode}
     687\end{ucppcode}&\begin{ucppcode}[tabsize=3]
    686688_Monitor Semaphore {
    687689
    … …  
    689691public:
    690692        void P() {
    691                 if(inUse) _Accept(V);
     693                if(inUse)
     694                        _Accept(V);
    692695                inUse = true;
    693696        }
    … …  
    697700        }
    698701}
    699 \end{ucppcode}
     702\end{ucppcode}&\begin{gocode}[tabsize=3]
     703type MySem struct {
     704        inUse bool
     705        c     chan bool
     706}
     707
     708// acquire
     709func (s MySem) P() {
     710        if s.inUse {
     711                select {
     712                case <-s.c:
     713                }
     714        }
     715        s.inUse = true
     716}
     717
     718// release
     719func (s MySem) V() {
     720        s.inUse = false
     721
     722        //This actually deadlocks
     723        //when single thread
     724        s.c <- false
     725}
     726\end{gocode}
    700727\end{tabular}
    701728\end{center}
    702 This method is more constrained and explicit, which may help users tone down the undeterministic nature of concurrency. Indeed, as the following examples demonstrates, external scheduling allows users to wait for events from other threads without the concern of unrelated events occuring. External scheduling can generally be done either in terms of control flow (e.g., \uC) or in terms of data (e.g. Go). Of course, both of these paradigms have their own strenghts and weaknesses but for this project control-flow semantics were chosen to stay consistent with the rest of the languages semantics. Two challenges specific to \CFA arise when trying to add external scheduling with loose object definitions and multi-monitor routines. The previous example shows a simple use \code{_Accept} versus \code{wait}/\code{signal} and its advantages. Note that while other languages often use \code{accept} as the core external scheduling keyword, \CFA uses \code{waitfor} to prevent name collisions with existing socket APIs.
    703 
    704 In the case of internal scheduling, the call to \code{wait} only guarantees that \code{V} is the last routine to access the monitor. This entails that the routine \code{V} may have acquired mutual exclusion several times while routine \code{P} was waiting. On the other hand, external scheduling guarantees that while routine \code{P} was waiting, no routine other than \code{V} could acquire the monitor.
     729This method is more constrained and explicit, which helps users tone down the undeterministic nature of concurrency. Indeed, as the following examples demonstrates, external scheduling allows users to wait for events from other threads without the concern of unrelated events occuring. External scheduling can generally be done either in terms of control flow (e.g., \uC with \code{_Accept}) or in terms of data (e.g., Go with channels). Of course, both of these paradigms have their own strenghts and weaknesses but for this project control-flow semantics were chosen to stay consistent with the rest of the languages semantics. Two challenges specific to \CFA arise when trying to add external scheduling with loose object definitions and multi-monitor routines. The previous example shows a simple use \code{_Accept} versus \code{wait}/\code{signal} and its advantages. Note that while other languages often use \code{accept}/\code{select} as the core external scheduling keyword, \CFA uses \code{waitfor} to prevent name collisions with existing socket \acrshort{api}s.
     730
     731For the \code{P} member above using internal scheduling, the call to \code{wait} only guarantees that \code{V} is the last routine to access the monitor, allowing a third routine, say \code{isInUse()}, acquire mutual exclusion several times while routine \code{P} is waiting. On the other hand, external scheduling guarantees that while routine \code{P} is waiting, no routine other than \code{V} can acquire the monitor.
    705732
    706733% ======================================================================
    … …  
    709736% ======================================================================
    710737% ======================================================================
    711 In \uC, monitor declarations include an exhaustive list of monitor operations. Since \CFA is not object oriented it becomes both more difficult to implement but also less clear for the user:
    712 
    713 \begin{cfacode}
    714         monitor A {};
    715 
    716         void f(A & mutex a);
    717         void f(int a, float b);
    718         void g(A & mutex a) {
    719                 waitfor(f); // Less obvious which f() to wait for
    720         }
     738In \uC, monitor declarations include an exhaustive list of monitor operations. Since \CFA is not object oriented, monitors become both more difficult to implement and less clear for a user:
     739
     740\begin{cfacode}
     741monitor A {};
     742
     743void f(A & mutex a);
     744void g(A & mutex a) {
     745        waitfor(f); //Obvious which f() to wait for
     746}
     747
     748void f(A & mutex a, int); //New different F added in scope
     749void h(A & mutex a) {
     750        waitfor(f); //Less obvious which f() to wait for
     751}
    721752\end{cfacode}
    722753
    … …  
    728759        if monitor is free
    729760                enter
    730         elif I already own the monitor
     761        elif already own the monitor
    731762                continue
    732763        elif monitor accepts me
    … …  
    738769\end{center}
    739770
    740 For the fist two conditions, it is easy to implement a check that can evaluate the condition in a few instruction. However, a fast check for \pscode{monitor accepts me} is much harder to implement depending on the constraints put on the monitors. Indeed, monitors are often expressed as an entry queue and some acceptor queue as in the following figure:
     771For the first two conditions, it is easy to implement a check that can evaluate the condition in a few instruction. However, a fast check for \pscode{monitor accepts me} is much harder to implement depending on the constraints put on the monitors. Indeed, monitors are often expressed as an entry queue and some acceptor queue as in the following figure:
    741772
    742773\begin{center}
    … …  
    744775\end{center}
    745776
    746 There are other alternatives to these pictures but in the case of this picture implementing a fast accept check is relatively easy. Indeed simply updating a bitmask when the acceptor queue changes is enough to have a check that executes in a single instruction, even with a fairly large number (e.g. 128) of mutex members. This technique cannot be used in \CFA because it relies on the fact that the monitor type declares all the acceptable routines. For OO languages this does not compromise much since monitors already have an exhaustive list of member routines. However, for \CFA this is not the case; routines can be added to a type anywhere after its declaration. Its important to note that the bitmask approach does not actually require an exhaustive list of routines, but it requires a dense unique ordering of routines with an upper-bound and that ordering must be consistent across translation units.
    747 The alternative would be to have a picture more like this one:
     777There are other alternatives to these pictures, but in the case of this picture, implementing a fast accept check is relatively easy. Restricted to a fixed number of mutex members, N, the accept check reduces to updating a bitmask when the acceptor queue changes, a check that executes in a single instruction even with a fairly large number (e.g., 128) of mutex members. This technique cannot be used in \CFA because it relies on the fact that the monitor type enumerates (declares) all the acceptable routines. For OO languages this does not compromise much since monitors already have an exhaustive list of member routines. However, for \CFA this is not the case; routines can be added to a type anywhere after its declaration. It is important to note that the bitmask approach does not actually require an exhaustive list of routines, but it requires a dense unique ordering of routines with an upper-bound and that ordering must be consistent across translation units.
     778The alternative is to alter the implementeation like this:
    748779
    749780\begin{center}
    … …  
    751782\end{center}
    752783
    753 Not storing the queues inside the monitor means that the storage can vary between routines, allowing for more flexibility and extensions. Storing an array of function-pointers would solve the issue of uniquely identifying acceptable routines. However, the single instruction bitmask compare has been replaced by dereferencing a pointer followed by a linear search. Furthermore, supporting nested external scheduling may now require additionnal searches on calls to waitfor to check if a routine is already queued in.
    754 
    755 At this point we must make a decision between flexibility and performance. Many design decisions in \CFA achieve both flexibility and performance, for example polymorphic routines add significant flexibility but inlining them means the optimizer can easily remove any runtime cost. Here however, the cost of flexibility cannot be trivially removed. In the end, the most flexible approach has been chosen since it allows users to write programs that would otherwise be prohibitively hard to write. This is based on the assumption that writing fast but inflexible locks is closer to a solved problems than writing locks that are as flexible as external scheduling in \CFA.
    756 
    757 Another aspect to consider is what happens if multiple overloads of the same routine are used. For the time being it is assumed that multiple overloads of the same routine are considered as distinct routines. However, this could easily be extended in the future.
     784Generating a mask dynamically means that the storage for the mask information can vary between calls to \code{waitfor}, allowing for more flexibility and extensions. Storing an array of accepted function-pointers replaces the single instruction bitmask compare with dereferencing a pointer followed by a linear search. Furthermore, supporting nested external scheduling (e.g., listing \ref{lst:nest-ext}) may now require additionnal searches on calls to \code{waitfor} statement to check if a routine is already queued in.
     785
     786\begin{figure}
     787\begin{cfacode}
     788monitor M {};
     789void foo( M & mutex a ) {}
     790void bar( M & mutex b ) {
     791        //Nested in the waitfor(bar, c) call
     792        waitfor(foo, b);
     793}
     794void baz( M & mutex c ) {
     795        waitfor(bar, c);
     796}
     797
     798\end{cfacode}
     799\caption{Example of nested external scheduling}
     800\label{lst:nest-ext}
     801\end{figure}
     802
     803Note that in the second picture, tasks need to always keep track of which routine they are attempting to acquire the monitor and the routine mask needs to have both a function pointer and a set of monitors, as will be discussed in the next section. These details where omitted from the picture for the sake of simplifying the representation.
     804
     805At this point, a decision must be made between flexibility and performance. Many design decisions in \CFA achieve both flexibility and performance, for example polymorphic routines add significant flexibility but inlining them means the optimizer can easily remove any runtime cost. Here however, the cost of flexibility cannot be trivially removed. In the end, the most flexible approach has been chosen since it allows users to write programs that would otherwise be prohibitively hard to write. This decision is based on the assumption that writing fast but inflexible locks is closer to a solved problems than writing locks that are as flexible as external scheduling in \CFA.
    758806
    759807% ======================================================================
    … …  
    763811% ======================================================================
    764812
    765 External scheduling, like internal scheduling, becomes orders of magnitude more complex when we start introducing multi-monitor syntax. Even in the simplest possible case some new semantics need to be established:
    766 \begin{cfacode}
    767         mutex struct A {};
    768 
    769         mutex struct B {};
    770 
    771         void g(A & mutex a, B & mutex b) {
    772                 waitfor(f); //ambiguous, which monitor
    773         }
     813External scheduling, like internal scheduling, becomes significantly more complex when introducing multi-monitor syntax. Even in the simplest possible case, some new semantics need to be established:
     814\begin{cfacode}
     815monitor M {};
     816
     817void f(M & mutex a);
     818
     819void g(M & mutex b, M & mutex c) {
     820        waitfor(f); //two monitors M => unkown which to pass to f(M & mutex)
     821}
    774822\end{cfacode}
    775823
    … …  
    777825
    778826\begin{cfacode}
    779         mutex struct A {};
    780 
    781         mutex struct B {};
    782 
    783         void g(A & mutex a, B & mutex b) {
    784                 waitfor( f, b );
    785         }
    786 \end{cfacode}
    787 
    788 This is unambiguous. Both locks will be acquired and kept, when routine \code{f} is called the lock for monitor \code{b} will be temporarily transferred from \code{g} to \code{f} (while \code{g} still holds lock \code{a}). This behavior can be extended to multi-monitor waitfor statment as follows.
    789 
    790 \begin{cfacode}
    791         mutex struct A {};
    792 
    793         mutex struct B {};
    794 
    795         void g(A & mutex a, B & mutex b) {
    796                 waitfor( f, a, b);
    797         }
    798 \end{cfacode}
    799 
    800 Note that the set of monitors passed to the \code{waitfor} statement must be entirely contained in the set of monitor already acquired in the routine. \code{waitfor} used in any other context is Undefined Behaviour.
    801 
    802 An important behavior to note is that what happens when set of monitors only match partially :
    803 
    804 \begin{cfacode}
    805         mutex struct A {};
    806 
    807         mutex struct B {};
    808 
    809         void g(A & mutex a, B & mutex b) {
    810                 waitfor(f, a, b);
    811         }
    812 
    813         A a1, a2;
    814         B b;
    815 
    816         void foo() {
    817                 g(a1, b);
    818         }
    819 
    820         void bar() {
    821                 f(a2, b);
    822         }
    823 \end{cfacode}
    824 
    825 While the equivalent can happen when using internal scheduling, the fact that conditions are branded on first use means that users have to use two different condition variables. In both cases, partially matching monitor sets will not wake-up the waiting thread. It is also important to note that in the case of external scheduling, as for routine calls, the order of parameters is important; \code{waitfor(f,a,b)} and \code{waitfor(f,b,a)} are to distinct waiting condition.
    826 
    827 % ======================================================================
    828 % ======================================================================
    829 \subsection{Implementation Details: External scheduling queues}
    830 % ======================================================================
    831 % ======================================================================
    832 To support multi-monitor external scheduling means that some kind of entry-queues must be used that is aware of both monitors. However, acceptable routines must be aware of the entry queues which means they must be stored inside at least one of the monitors that will be acquired. This in turn adds the requirement a systematic algorithm of disambiguating which queue is relavant regardless of user ordering. The proposed algorithm is to fall back on monitors lock ordering and specify that the monitor that is acquired first is the lock with the relevant entry queue. This assumes that the lock acquiring order is static for the lifetime of all concerned objects but that is a reasonable constraint. This algorithm choice has two consequences, the entry queue of the highest priority monitor is no longer a true FIFO queue and the queue of the lowest priority monitor is both required and probably unused. The queue can no longer be a FIFO queue because instead of simply containing the waiting threads in order arrival, they also contain the second mutex. Therefore, another thread with the same highest priority monitor but a different lowest priority monitor may arrive first but enter the critical section after a thread with the correct pairing. Secondly, since it may not be known at compile time which monitor will be the lowest priority monitor, every monitor needs to have the correct queues even though it is probable that half the multi-monitor queues will go unused for the entire duration of the program.
    833 
    834 % ======================================================================
    835 % ======================================================================
    836 \section{Other concurrency tools}
    837 % ======================================================================
    838 % ======================================================================
    839 % \TODO
     827monitor M {};
     828
     829void f(M & mutex a);
     830
     831void g(M & mutex a, M & mutex b) {
     832        waitfor( f, b );
     833}
     834\end{cfacode}
     835
     836This syntax is unambiguous. Both locks are acquired and kept by \code{g}. When routine \code{f} is called, the lock for monitor \code{b} is temporarily transferred from \code{g} to \code{f} (while \code{g} still holds lock \code{a}). This behavior can be extended to multi-monitor \code{waitfor} statement as follows.
     837
     838\begin{cfacode}
     839monitor M {};
     840
     841void f(M & mutex a, M & mutex b);
     842
     843void g(M & mutex a, M & mutex b) {
     844        waitfor( f, a, b);
     845}
     846\end{cfacode}
     847
     848Note that the set of monitors passed to the \code{waitfor} statement must be entirely contained in the set of monitors already acquired in the routine. \code{waitfor} used in any other context is Undefined Behaviour.
     849
     850An important behavior to note is when a set of monitors only match partially :
     851
     852\begin{cfacode}
     853mutex struct A {};
     854
     855mutex struct B {};
     856
     857void g(A & mutex a, B & mutex b) {
     858        waitfor(f, a, b);
     859}
     860
     861A a1, a2;
     862B b;
     863
     864void foo() {
     865        g(a1, b); //block on accept
     866}
     867
     868void bar() {
     869        f(a2, b); //fufill cooperation
     870}
     871\end{cfacode}
     872
     873While the equivalent can happen when using internal scheduling, the fact that conditions are specific to a set of monitors means that users have to use two different condition variables. In both cases, partially matching monitor sets does not wake-up the waiting thread. It is also important to note that in the case of external scheduling, as for routine calls, the order of parameters is irrelevant; \code{waitfor(f,a,b)} and \code{waitfor(f,b,a)} are indistinguishable waiting condition.
     874
     875% ======================================================================
     876% ======================================================================
     877\subsection{\code{waitfor} semantics}
     878% ======================================================================
     879% ======================================================================
     880
     881Syntactically, the \code{waitfor} statement takes a function identifier and a set of monitors. While the set of monitors can be any list of expression, the function name is more restricted because the compiler validates at compile time the validity of the function type and the parameters used with the \code{waitfor} statement. It checks that the set of monitor passed in matches the requirements for a function call. Listing \ref{lst:waitfor} shows various usage of the waitfor statement and which are acceptable. The choice of the function type is made ignoring any non-\code{mutex} parameter. One limitation of the current implementation is that it does not handle overloading.
     882\begin{figure}
     883\begin{cfacode}
     884monitor A{};
     885monitor B{};
     886
     887void f1( A & mutex );
     888void f2( A & mutex, B & mutex );
     889void f3( A & mutex, int );
     890void f4( A & mutex, int );
     891void f4( A & mutex, double );
     892
     893void foo( A & mutex a1, A & mutex a2, B & mutex b1, B & b2 ) {
     894        A * ap = & a1;
     895        void (*fp)( A & mutex ) = f1;
     896
     897        waitfor(f1, a1);     //Correct : 1 monitor case
     898        waitfor(f2, a1, b1); //Correct : 2 monitor case
     899        waitfor(f3, a1);     //Correct : non-mutex arguments are ignored
     900        waitfor(f1, *ap);    //Correct : expression as argument
     901
     902        waitfor(f1, a1, b1); //Incorrect : Too many mutex arguments
     903        waitfor(f2, a1);     //Incorrect : Too few mutex arguments
     904        waitfor(f2, a1, a2); //Incorrect : Mutex arguments don't match
     905        waitfor(f1, 1);      //Incorrect : 1 not a mutex argument
     906        waitfor(f9, a1);     //Incorrect : f9 function does not exist
     907        waitfor(*fp, a1 );   //Incorrect : fp not an identifier
     908        waitfor(f4, a1);     //Incorrect : f4 ambiguous
     909
     910        waitfor(f2, a1, b2); //Undefined Behaviour : b2 may not acquired
     911}
     912\end{cfacode}
     913\caption{Various correct and incorrect uses of the waitfor statement}
     914\label{lst:waitfor}
     915\end{figure}
     916
     917Finally, for added flexibility, \CFA supports constructing complex \code{waitfor} mask using the \code{or}, \code{timeout} and \code{else}. Indeed, multiple \code{waitfor} can be chained together using \code{or}; this chain forms a single statement that uses baton-pass to any one function that fits one of the function+monitor set passed in. To eanble users to tell which accepted function is accepted, \code{waitfor}s are followed by a statement (including the null statement \code{;}) or a compound statement. When multiple \code{waitfor} are chained together, only the statement corresponding to the accepted function is executed. A \code{waitfor} chain can also be followed by a \code{timeout}, to signify an upper bound on the wait, or an \code{else}, to signify that the call should be non-blocking, that is only check of a matching function call already arrived and return immediately otherwise. Any and all of these clauses can be preceded by a \code{when} condition to dynamically construct the mask based on some current state. Listing \ref{lst:waitfor2}, demonstrates several complex masks and some incorrect ones.
     918
     919\begin{figure}
     920\begin{cfacode}
     921monitor A{};
     922
     923void f1( A & mutex );
     924void f2( A & mutex );
     925
     926void foo( A & mutex a, bool b, int t ) {
     927        //Correct : blocking case
     928        waitfor(f1, a);
     929
     930        //Correct : block with statement
     931        waitfor(f1, a) {
     932                sout | "f1" | endl;
     933        }
     934
     935        //Correct : block waiting for f1 or f2
     936        waitfor(f1, a) {
     937                sout | "f1" | endl;
     938        } or waitfor(f2, a) {
     939                sout | "f2" | endl;
     940        }
     941
     942        //Correct : non-blocking case
     943        waitfor(f1, a); or else;
     944
     945        //Correct : non-blocking case
     946        waitfor(f1, a) {
     947                sout | "blocked" | endl;
     948        } or else {
     949                sout | "didn't block" | endl;
     950        }
     951
     952        //Correct : block at most 10 seconds
     953        waitfor(f1, a) {
     954                sout | "blocked" | endl;
     955        } or timeout( 10`s) {
     956                sout | "didn't block" | endl;
     957        }
     958
     959        //Correct : block only if b == true
     960        //if b == false, don't even make the call
     961        when(b) waitfor(f1, a);
     962
     963        //Correct : block only if b == true
     964        //if b == false, make non-blocking call
     965        waitfor(f1, a); or when(!b) else;
     966
     967        //Correct : block only of t > 1
     968        waitfor(f1, a); or when(t > 1) timeout(t); or else;
     969
     970        //Incorrect : timeout clause is dead code
     971        waitfor(f1, a); or timeout(t); or else;
     972
     973        //Incorrect : order must be
     974        //waitfor [or waitfor... [or timeout] [or else]]
     975        timeout(t); or waitfor(f1, a); or else;
     976}
     977\end{cfacode}
     978\caption{Various correct and incorrect uses of the or, else, and timeout clause around a waitfor statement}
     979\label{lst:waitfor2}
     980\end{figure}
     981
     982% ======================================================================
     983% ======================================================================
     984\subsection{Waiting for the destructor}
     985% ======================================================================
     986% ======================================================================
     987An interesting use for the \code{waitfor} statement is destructor semantics. Indeed, the \code{waitfor} statement can accept any \code{mutex} routine, which includes the destructor (see section \ref{data}). However, with the semantics discussed until now, waiting for the destructor does not make any sense since using an object after its destructor is called is undefined behaviour. The simplest approach is to disallow \code{waitfor} on a destructor. However, a more expressive approach is to flip execution ordering when waiting for the destructor, meaning that waiting for the destructor allows the destructor to run after the current \code{mutex} routine, similarly to how a condition is signalled.
     988\begin{figure}
     989\begin{cfacode}
     990monitor Executer {};
     991struct  Action;
     992
     993void ^?{}   (Executer & mutex this);
     994void execute(Executer & mutex this, const Action & );
     995void run    (Executer & mutex this) {
     996        while(true) {
     997                   waitfor(execute, this);
     998                or waitfor(^?{}   , this) {
     999                        break;
     1000                }
     1001        }
     1002}
     1003\end{cfacode}
     1004\caption{Example of an executor which executes action in series until the destructor is called.}
     1005\label{lst:dtor-order}
     1006\end{figure}
     1007For example, listing \ref{lst:dtor-order} shows an example of an executor with an infinite loop, which waits for the destructor to break out of this loop. Switching the semantic meaning introduces an idiomatic way to terminate a task and/or wait for its termination via destruction.
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