source: libcfa/src/concurrency/select.hfa@ fc95df3

//
// Cforall Version 1.0.0 Copyright (C) 2021 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// channel.hfa -- LIBCFATHREAD
// Runtime locks that used with the runtime thread system.
//
// Author           : Colby Alexander Parsons
// Created On       : Thu Jan 21 19:46:50 2023
// Last Modified By :
// Last Modified On :
// Update Count     :
//

#pragma once

#include "containers/list.hfa"
#include "alarm.hfa"
#include "kernel.hfa"
#include "time.hfa"

struct select_node;

// node status
static const unsigned long int __SELECT_UNSAT = 0;
static const unsigned long int __SELECT_PENDING = 1; // used only by special OR case
static const unsigned long int __SELECT_SAT = 2;
static const unsigned long int __SELECT_RUN = 3;

// these are used inside the compiler to aid in code generation
static inline bool __CFA_has_clause_run( unsigned long int status ) { return status == __SELECT_RUN; }
static inline void __CFA_maybe_park( int * park_counter ) {
    if ( __atomic_sub_fetch( park_counter, 1, __ATOMIC_SEQ_CST) < 0 )
        park();
}

// node used for coordinating waituntil synchronization
struct select_node {
    int * park_counter;                 // If this is 0p then the node is in a special OR case waituntil
    unsigned long int * clause_status;  // needs to point at ptr sized location, if this is 0p then node is not part of a waituntil

    void * extra;                       // used to store arbitrary data needed by some primitives

    thread$ * blocked_thread;
    inline dlink(select_node);
};
P9_EMBEDDED( select_node, dlink(select_node) )

static inline void ?{}( select_node & this ) {
    this.blocked_thread = active_thread();
    this.clause_status = 0p;
    this.park_counter = 0p;
    this.extra = 0p;
}

static inline void ?{}( select_node & this, thread$ * blocked_thread ) {
    this.blocked_thread = blocked_thread;
    this.clause_status = 0p;
    this.park_counter = 0p;
    this.extra = 0p;
}

static inline void ?{}( select_node & this, thread$ * blocked_thread, void * extra ) {
    this.blocked_thread = blocked_thread;
    this.clause_status = 0p;
    this.park_counter = 0p;
    this.extra = extra;
}
static inline void ^?{}( select_node & this ) {}

// this is used inside the compiler to aid in code generation
static inline unsigned long int * __get_clause_status( select_node & s ) { return s.clause_status; }

// this is used inside the compiler to attempt to establish an else clause as a winner in the OR special case race
static inline bool __select_node_else_race( select_node & this ) with( this ) {
    unsigned long int cmp_status = __SELECT_UNSAT;
    return *clause_status == 0 
            && __atomic_compare_exchange_n( clause_status, &cmp_status, __SELECT_SAT, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST );
}

//-----------------------------------------------------------------------------
// is_selectable
forall(T & | sized(T))
trait is_selectable {
    // For registering a select stmt on a selectable concurrency primitive
    // Returns bool that indicates if operation is already SAT
    bool register_select( T &, select_node & );

    // For unregistering a select stmt on a selectable concurrency primitive
    // If true is returned then the corresponding code block is run (only in non-special OR case and only if node status is not RUN)
    bool unregister_select( T &, select_node & );

    // This routine is run on the selecting thread prior to executing the statement corresponding to the select_node
    //    passed as an arg to this routine
    // If on_selected returns false, the statement is not run if the on_selected call occured as 
    //    part of a statement block that only ran due to an unregister_select that returned true
    //    In all other executions the return value is ignored.
    bool on_selected( T &, select_node & );
};

//=============================================================================================
// Waituntil Helpers
//=============================================================================================

static inline void __make_select_node_unsat( select_node & this ) with( this ) {
    __atomic_store_n( clause_status, __SELECT_UNSAT, __ATOMIC_SEQ_CST );
}
static inline void __make_select_node_sat( select_node & this ) with( this ) {
    __atomic_store_n( clause_status, __SELECT_SAT, __ATOMIC_SEQ_CST );
}

// used for the 2-stage avail needed by the special OR case
static inline bool __mark_select_node( select_node & this, unsigned long int val ) with( this ) {
    /* paranoid */ verify( park_counter == 0p );
    /* paranoid */ verify( clause_status != 0p );

    unsigned long int cmp_status = __SELECT_UNSAT;
    while( !__atomic_compare_exchange_n( clause_status, &cmp_status, val, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST ) ) {
        if ( cmp_status != __SELECT_PENDING ) return false;
        cmp_status = __SELECT_UNSAT;
    }
    return true;
}

// used for the 2-stage avail by the thread who owns a pending node
static inline bool __pending_set_other( select_node & other, select_node & mine, unsigned long int val ) with( other ) {
    /* paranoid */ verify( park_counter == 0p );
    /* paranoid */ verify( clause_status != 0p );

    unsigned long int cmp_status = __SELECT_UNSAT;
    while( !__atomic_compare_exchange_n( clause_status, &cmp_status, val, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST ) ) {
        if ( cmp_status != __SELECT_PENDING )
            return false;

        // toggle current status flag to avoid starvation/deadlock
        __make_select_node_unsat( mine );
        cmp_status = __SELECT_UNSAT;
        if ( !__atomic_compare_exchange_n( mine.clause_status, &cmp_status, __SELECT_PENDING, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST ) )
            return false;
        cmp_status = __SELECT_UNSAT;
    }
    return true;
}

static inline bool __make_select_node_pending( select_node & this ) with( this ) {
    return __mark_select_node( this, __SELECT_PENDING );
}

// when a primitive becomes available it calls the following routine on it's node to update the select state:
// return true if we want to unpark the thd
static inline bool __make_select_node_available( select_node & this ) with( this ) {
    /* paranoid */ verify( clause_status != 0p );
    if( !park_counter )
        return __mark_select_node( this, (unsigned long int)&this );

    unsigned long int cmp_status = __SELECT_UNSAT;

    return *clause_status == 0 // C_TODO might not need a cmp_xchg in non special OR case
        && __atomic_compare_exchange_n( clause_status, &cmp_status, __SELECT_SAT, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST ) // can maybe just use atomic write
        && !__atomic_add_fetch( park_counter, 1, __ATOMIC_SEQ_CST);
}

// Handles the special OR case of the waituntil statement
// Since only one select node can win in the OR case, we need to race to set the node available BEFORE
//    performing the operation since if we lose the race the operation should not be performed as it will be lost
// Returns true if execution can continue normally and false if the queue has now been drained
static inline bool __handle_waituntil_OR( dlist( select_node ) & queue ) {
    if ( queue`isEmpty ) return false;
    if ( queue`first.clause_status && !queue`first.park_counter ) {
        while ( !queue`isEmpty ) {
            // if node not a special OR case or if we win the special OR case race break
            if ( !queue`first.clause_status || queue`first.park_counter || __make_select_node_available( queue`first ) )
                return true;
            // otherwise we lost the special OR race so discard node
            try_pop_front( queue );
        }
        return false;
    }
    return true;
}

// wake one thread from the list
static inline void wake_one( dlist( select_node ) & queue, select_node & popped ) {
    if ( !popped.clause_status                              // normal case, node is not a select node
        || ( popped.clause_status && !popped.park_counter ) // If popped link is special case OR selecting unpark but don't call __make_select_node_available
        || __make_select_node_available( popped ) )         // check if popped link belongs to a selecting thread
        unpark( popped.blocked_thread );
}

static inline void wake_one( dlist( select_node ) & queue ) { wake_one( queue, try_pop_front( queue ) ); }

static inline void setup_clause( select_node & this, unsigned long int * clause_status, int * park_counter ) {
    this.blocked_thread = active_thread();
    this.clause_status = clause_status;
    this.park_counter = park_counter;
}

// waituntil ( timeout( ... ) ) support
struct select_timeout_node {
    alarm_node_t a_node;
    select_node * s_node;
};
void ?{}( select_timeout_node & this, Duration duration, Alarm_Callback callback );
void ^?{}( select_timeout_node & this );
void timeout_handler_select_cast( alarm_node_t & node );

// Selectable trait routines
bool register_select( select_timeout_node & this, select_node & node );
bool unregister_select( select_timeout_node & this, select_node & node );
bool on_selected( select_timeout_node & this, select_node & node );

// Gateway routines to waituntil on duration
select_timeout_node timeout( Duration duration );
select_timeout_node sleep( Duration duration );