source: src/libcfa/concurrency/monitor.c@ 5363fdf

//
// Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// monitor_desc.c --
//
// Author           : Thierry Delisle
// Created On       : Thd Feb 23 12:27:26 2017
// Last Modified By : Peter A. Buhr
// Last Modified On : Mon Jul 31 14:59:05 2017
// Update Count     : 3
//

#include "monitor"

#include <stdlib>

#include "libhdr.h"
#include "kernel_private.h"

//-----------------------------------------------------------------------------
// Forward declarations
static inline void set_owner ( monitor_desc * this, thread_desc * owner );
static inline void set_owner ( monitor_desc ** storage, short count, thread_desc * owner );
static inline void set_mask  ( monitor_desc ** storage, short count, const __waitfor_mask_t & mask );
static inline void reset_mask( monitor_desc * this );

static inline thread_desc * next_thread( monitor_desc * this );
static inline bool is_accepted( monitor_desc * this, const __monitor_group_t & monitors );

static inline void lock_all( spinlock ** locks, unsigned short count );
static inline void lock_all( monitor_desc ** source, spinlock ** /*out*/ locks, unsigned short count );
static inline void unlock_all( spinlock ** locks, unsigned short count );
static inline void unlock_all( monitor_desc ** locks, unsigned short count );

static inline void save   ( monitor_desc ** ctx, short count, spinlock ** locks, unsigned int * /*out*/ recursions, __waitfor_mask_t * /*out*/ masks );
static inline void restore( monitor_desc ** ctx, short count, spinlock ** locks, unsigned int * /*in */ recursions, __waitfor_mask_t * /*in */ masks );

static inline void init     ( int count, monitor_desc ** monitors, __condition_node_t * waiter, __condition_criterion_t * criteria );
static inline void init_push( int count, monitor_desc ** monitors, __condition_node_t * waiter, __condition_criterion_t * criteria );

static inline thread_desc *        check_condition   ( __condition_criterion_t * );
static inline void                 brand_condition   ( condition * );
static inline [thread_desc *, int] search_entry_queue( const __waitfor_mask_t &, monitor_desc ** monitors, int count );

forall(dtype T | sized( T ))
static inline short insert_unique( T ** array, short & size, T * val );
static inline short count_max    ( const __waitfor_mask_t & mask );
static inline short aggregate    ( monitor_desc ** storage, const __waitfor_mask_t & mask );

//-----------------------------------------------------------------------------
// Useful defines
#define wait_ctx(thrd, user_info)                               /* Create the necessary information to use the signaller stack                         */ \
        __condition_node_t waiter = { thrd, count, user_info };   /* Create the node specific to this wait operation                                     */ \
        __condition_criterion_t criteria[count];                  /* Create the creteria this wait operation needs to wake up                            */ \
        init( count, monitors, &waiter, criteria );               /* Link everything together                                                            */ \

#define wait_ctx_primed(thrd, user_info)                        /* Create the necessary information to use the signaller stack                         */ \
        __condition_node_t waiter = { thrd, count, user_info };   /* Create the node specific to this wait operation                                     */ \
        __condition_criterion_t criteria[count];                  /* Create the creteria this wait operation needs to wake up                            */ \
        init_push( count, monitors, &waiter, criteria );          /* Link everything together and push it to the AS-Stack                                */ \

#define monitor_ctx( mons, cnt )                                /* Define that create the necessary struct for internal/external scheduling operations */ \
        monitor_desc ** monitors = mons;                          /* Save the targeted monitors                                                          */ \
        unsigned short count = cnt;                               /* Save the count to a local variable                                                  */ \
        unsigned int recursions[ count ];                         /* Save the current recursion levels to restore them later                             */ \
        __waitfor_mask_t masks[ count ];                          /* Save the current waitfor masks to restore them later                                */ \
        spinlock *   locks     [ count ];                         /* We need to pass-in an array of locks to BlockInternal                               */ \

#define monitor_save    save   ( monitors, count, locks, recursions, masks )
#define monitor_restore restore( monitors, count, locks, recursions, masks )


//-----------------------------------------------------------------------------
// Enter/Leave routines


extern "C" {
        // Enter single monitor
        static void __enter_monitor_desc( monitor_desc * this, const __monitor_group_t & group ) {
                // Lock the monitor spinlock, lock_yield to reduce contention
                lock_yield( &this->lock DEBUG_CTX2 );
                thread_desc * thrd = this_thread;

                LIB_DEBUG_PRINT_SAFE("Kernel : %10p Entering mon %p (%p)\n", thrd, this, this->owner);

                if( !this->owner ) {
                        // No one has the monitor, just take it
                        set_owner( this, thrd );

                        LIB_DEBUG_PRINT_SAFE("Kernel :  mon is free \n");
                }
                else if( this->owner == thrd) {
                        // We already have the monitor, just not how many times we took it
                        verify( this->recursion > 0 );
                        this->recursion += 1;

                        LIB_DEBUG_PRINT_SAFE("Kernel :  mon already owned \n");
                }
                else if( is_accepted( this, group) ) {
                        // Some one was waiting for us, enter
                        set_owner( this, thrd );

                        // Reset mask
                        reset_mask( this );

                        LIB_DEBUG_PRINT_SAFE("Kernel :  mon accepts \n");
                }
                else {
                        LIB_DEBUG_PRINT_SAFE("Kernel :  blocking \n");

                        // Some one else has the monitor, wait in line for it
                        append( &this->entry_queue, thrd );
                        BlockInternal( &this->lock );

                        LIB_DEBUG_PRINT_SAFE("Kernel : %10p Entered  mon %p\n", thrd, this);

                        // BlockInternal will unlock spinlock, no need to unlock ourselves
                        return;
                }

                LIB_DEBUG_PRINT_SAFE("Kernel : %10p Entered  mon %p\n", thrd, this);

                // Release the lock and leave
                unlock( &this->lock );
                return;
        }

        // Leave single monitor
        void __leave_monitor_desc( monitor_desc * this ) {
                // Lock the monitor spinlock, lock_yield to reduce contention
                lock_yield( &this->lock DEBUG_CTX2 );

                LIB_DEBUG_PRINT_SAFE("Kernel : %10p Leaving mon %p (%p)\n", this_thread, this, this->owner);

                verifyf( this_thread == this->owner, "Expected owner to be %p, got %p (r: %i, m: %p)", this_thread, this->owner, this->recursion, this );

                // Leaving a recursion level, decrement the counter
                this->recursion -= 1;

                // If we haven't left the last level of recursion
                // it means we don't need to do anything
                if( this->recursion != 0) {
                        unlock( &this->lock );
                        return;
                }

                // Get the next thread, will be null on low contention monitor
                thread_desc * new_owner = next_thread( this );

                // We can now let other threads in safely
                unlock( &this->lock );

                //We need to wake-up the thread
                WakeThread( new_owner );
        }

        // Leave the thread monitor
        // last routine called by a thread.
        // Should never return
        void __leave_thread_monitor( thread_desc * thrd ) {
                monitor_desc * this = &thrd->self_mon;

                // Lock the monitor now
                lock_yield( &this->lock DEBUG_CTX2 );

                disable_interrupts();

                thrd->self_cor.state = Halted;

                verifyf( thrd == this->owner, "Expected owner to be %p, got %p (r: %i, m: %p)", thrd, this->owner, this->recursion, this );

                // Leaving a recursion level, decrement the counter
                this->recursion -= 1;

                // If we haven't left the last level of recursion
                // it must mean there is an error
                if( this->recursion != 0) { abortf("Thread internal monitor has unbalanced recursion"); }

                // Fetch the next thread, can be null
                thread_desc * new_owner = next_thread( this );

                // Leave the thread, this will unlock the spinlock
                // Use leave thread instead of BlockInternal which is
                // specialized for this case and supports null new_owner
                LeaveThread( &this->lock, new_owner );

                // Control flow should never reach here!
        }
}

// Enter multiple monitor
// relies on the monitor array being sorted
static inline void enter( __monitor_group_t monitors ) {
        for(int i = 0; i < monitors.size; i++) {
                __enter_monitor_desc( monitors.list[i], monitors );
        }
}

// Leave multiple monitor
// relies on the monitor array being sorted
static inline void leave(monitor_desc ** monitors, int count) {
        for(int i = count - 1; i >= 0; i--) {
                __leave_monitor_desc( monitors[i] );
        }
}

// Ctor for monitor guard
// Sorts monitors before entering
void ?{}( monitor_guard_t & this, monitor_desc ** m, int count, void (*func)() ) {
        // Store current array
        this.m = m;
        this.count = count;

        // Sort monitors based on address -> TODO use a sort specialized for small numbers
        qsort(this.m, count);

        // Save previous thread context
        this.prev_mntrs = this_thread->monitors.list;
        this.prev_count = this_thread->monitors.size;
        this.prev_func  = this_thread->monitors.func;

        // Update thread context (needed for conditions)
        this_thread->monitors.list = m;
        this_thread->monitors.size = count;
        this_thread->monitors.func = func;

        // LIB_DEBUG_PRINT_SAFE("MGUARD : enter %d\n", count);

        // Enter the monitors in order
        __monitor_group_t group = {this.m, this.count, func};
        enter( group );

        // LIB_DEBUG_PRINT_SAFE("MGUARD : entered\n");
}


// Dtor for monitor guard
void ^?{}( monitor_guard_t & this ) {
        // LIB_DEBUG_PRINT_SAFE("MGUARD : leaving %d\n", this.count);

        // Leave the monitors in order
        leave( this.m, this.count );

        // LIB_DEBUG_PRINT_SAFE("MGUARD : left\n");

        // Restore thread context
        this_thread->monitors.list = this.prev_mntrs;
        this_thread->monitors.size = this.prev_count;
        this_thread->monitors.func = this.prev_func;
}

//-----------------------------------------------------------------------------
// Internal scheduling types
void ?{}(__condition_node_t & this, thread_desc * waiting_thread, unsigned short count, uintptr_t user_info ) {
        this.waiting_thread = waiting_thread;
        this.count = count;
        this.next = NULL;
        this.user_info = user_info;
}

void ?{}(__condition_criterion_t & this ) {
        this.ready  = false;
        this.target = NULL;
        this.owner  = NULL;
        this.next   = NULL;
}

void ?{}(__condition_criterion_t & this, monitor_desc * target, __condition_node_t * owner ) {
        this.ready  = false;
        this.target = target;
        this.owner  = owner;
        this.next   = NULL;
}

//-----------------------------------------------------------------------------
// Internal scheduling
void wait( condition * this, uintptr_t user_info = 0 ) {
        brand_condition( this );

        // Check that everything is as expected
        assertf( this->monitors != NULL, "Waiting with no monitors (%p)", this->monitors );
        verifyf( this->monitor_count != 0, "Waiting with 0 monitors (%i)", this->monitor_count );
        verifyf( this->monitor_count < 32u, "Excessive monitor count (%i)", this->monitor_count );

        // Create storage for monitor context
        monitor_ctx( this->monitors, this->monitor_count );

        // Create the node specific to this wait operation
        wait_ctx( this_thread, user_info );

        // Append the current wait operation to the ones already queued on the condition
        // We don't need locks for that since conditions must always be waited on inside monitor mutual exclusion
        append( &this->blocked, &waiter );

        // Lock all monitors (aggregates the locks as well)
        lock_all( monitors, locks, count );

        // Find the next thread(s) to run
        short thread_count = 0;
        thread_desc * threads[ count ];
        for(int i = 0; i < count; i++) {
                threads[i] = 0;
        }

        // Save monitor states
        monitor_save;

        // Remove any duplicate threads
        for( int i = 0; i < count; i++) {
                thread_desc * new_owner = next_thread( monitors[i] );
                insert_unique( threads, thread_count, new_owner );
        }

        // Everything is ready to go to sleep
        BlockInternal( locks, count, threads, thread_count );

        // We are back, restore the owners and recursions
        monitor_restore;
}

bool signal( condition * this ) {
        if( is_empty( this ) ) { return false; }

        //Check that everything is as expected
        verify( this->monitors );
        verify( this->monitor_count != 0 );

        //Some more checking in debug
        LIB_DEBUG_DO(
                thread_desc * this_thrd = this_thread;
                if ( this->monitor_count != this_thrd->monitors.size ) {
                        abortf( "Signal on condition %p made with different number of monitor(s), expected %i got %i", this, this->monitor_count, this_thrd->monitors.size );
                }

                for(int i = 0; i < this->monitor_count; i++) {
                        if ( this->monitors[i] != this_thrd->monitors.list[i] ) {
                                abortf( "Signal on condition %p made with different monitor, expected %p got %i", this, this->monitors[i], this_thrd->monitors.list[i] );
                        }
                }
        );

        unsigned short count = this->monitor_count;

        // Lock all monitors
        lock_all( this->monitors, NULL, count );

        //Pop the head of the waiting queue
        __condition_node_t * node = pop_head( &this->blocked );

        //Add the thread to the proper AS stack
        for(int i = 0; i < count; i++) {
                __condition_criterion_t * crit = &node->criteria[i];
                assert( !crit->ready );
                push( &crit->target->signal_stack, crit );
        }

        //Release
        unlock_all( this->monitors, count );

        return true;
}

bool signal_block( condition * this ) {
        if( !this->blocked.head ) { return false; }

        //Check that everything is as expected
        verifyf( this->monitors != NULL, "Waiting with no monitors (%p)", this->monitors );
        verifyf( this->monitor_count != 0, "Waiting with 0 monitors (%i)", this->monitor_count );

        // Create storage for monitor context
        monitor_ctx( this->monitors, this->monitor_count );

        // Lock all monitors (aggregates the locks them as well)
        lock_all( monitors, locks, count );

        // Create the node specific to this wait operation
        wait_ctx_primed( this_thread, 0 )

        //save contexts
        monitor_save;

        //Find the thread to run
        thread_desc * signallee = pop_head( &this->blocked )->waiting_thread;
        set_owner( monitors, count, signallee );

        //Everything is ready to go to sleep
        BlockInternal( locks, count, &signallee, 1 );


        // WE WOKE UP


        //We are back, restore the masks and recursions
        monitor_restore;

        return true;
}

// Access the user_info of the thread waiting at the front of the queue
uintptr_t front( condition * this ) {
        verifyf( !is_empty(this),
                "Attempt to access user data on an empty condition.\n"
                "Possible cause is not checking if the condition is empty before reading stored data."
        );
        return this->blocked.head->user_info;
}

//-----------------------------------------------------------------------------
// External scheduling
// cases to handle :
//      - target already there :
//              block and wake
//      - dtor already there
//              put thread on signaller stack
//      - non-blocking
//              return else
//      - timeout
//              return timeout
//      - block
//              setup mask
//              block
void __waitfor_internal( const __waitfor_mask_t & mask, int duration ) {
        // This statment doesn't have a contiguous list of monitors...
        // Create one!
        short max = count_max( mask );
        monitor_desc * mon_storage[max];
        short actual_count = aggregate( mon_storage, mask );

        if(actual_count == 0) return;

        LIB_DEBUG_PRINT_SAFE("Kernel : waitfor internal proceeding\n");

        // Create storage for monitor context
        monitor_ctx( mon_storage, actual_count );

        // Lock all monitors (aggregates the locks as well)
        lock_all( monitors, locks, count );

        {
                // Check if the entry queue
                thread_desc * next; int index;
                [next, index] = search_entry_queue( mask, monitors, count );

                if( next ) {
                        *mask.accepted = index;
                        if( mask.clauses[index].is_dtor ) {
                                #warning case not implemented
                        }
                        else {
                                LIB_DEBUG_PRINT_SAFE("Kernel : thread present, baton-passing\n");

                                // Create the node specific to this wait operation
                                wait_ctx_primed( this_thread, 0 );

                                // Save monitor states
                                monitor_save;

                                // Set the owners to be the next thread
                                set_owner( monitors, count, next );

                                // Everything is ready to go to sleep
                                BlockInternal( locks, count, &next, 1 );

                                // We are back, restore the owners and recursions
                                monitor_restore;

                                LIB_DEBUG_PRINT_SAFE("Kernel : thread present, returned\n");
                        }

                        LIB_DEBUG_PRINT_SAFE("Kernel : accepted %d\n", *mask.accepted);

                        return;
                }
        }


        if( duration == 0 ) {
                LIB_DEBUG_PRINT_SAFE("Kernel : non-blocking, exiting\n");

                unlock_all( locks, count );

                LIB_DEBUG_PRINT_SAFE("Kernel : accepted %d\n", *mask.accepted);
                return;
        }


        verifyf( duration < 0, "Timeout on waitfor statments not supported yet.");

        LIB_DEBUG_PRINT_SAFE("Kernel : blocking waitfor\n");

        // Create the node specific to this wait operation
        wait_ctx_primed( this_thread, 0 );

        monitor_save;
        set_mask( monitors, count, mask );

        for(int i = 0; i < count; i++) {
                verify( monitors[i]->owner == this_thread );
        }

        //Everything is ready to go to sleep
        BlockInternal( locks, count );


        // WE WOKE UP


        //We are back, restore the masks and recursions
        monitor_restore;

        LIB_DEBUG_PRINT_SAFE("Kernel : exiting\n");

        LIB_DEBUG_PRINT_SAFE("Kernel : accepted %d\n", *mask.accepted);
}

//-----------------------------------------------------------------------------
// Utilities

static inline void set_owner( monitor_desc * this, thread_desc * owner ) {
        // LIB_DEBUG_PRINT_SAFE("Kernal :   Setting owner of %p to %p ( was %p)\n", this, owner, this->owner );

        //Pass the monitor appropriately
        this->owner = owner;

        //We are passing the monitor to someone else, which means recursion level is not 0
        this->recursion = owner ? 1 : 0;
}

static inline void set_owner( monitor_desc ** monitors, short count, thread_desc * owner ) {
        for( int i = 0; i < count; i++ ) {
                set_owner( monitors[i], owner );
        }
}

static inline void set_mask( monitor_desc ** storage, short count, const __waitfor_mask_t & mask ) {
        for(int i = 0; i < count; i++) {
                storage[i]->mask = mask;
        }
}

static inline void reset_mask( monitor_desc * this ) {
        this->mask.accepted = NULL;
        this->mask.clauses = NULL;
        this->mask.size = 0;
}

static inline thread_desc * next_thread( monitor_desc * this ) {
        //Check the signaller stack
        __condition_criterion_t * urgent = pop( &this->signal_stack );
        if( urgent ) {
                //The signaller stack is not empty,
                //regardless of if we are ready to baton pass,
                //we need to set the monitor as in use
                set_owner( this,  urgent->owner->waiting_thread );

                return check_condition( urgent );
        }

        // No signaller thread
        // Get the next thread in the entry_queue
        thread_desc * new_owner = pop_head( &this->entry_queue );
        set_owner( this, new_owner );

        return new_owner;
}

static inline bool is_accepted( monitor_desc * this, const __monitor_group_t & group ) {
        __acceptable_t * it = this->mask.clauses; // Optim
        int count = this->mask.size;

        // Check if there are any acceptable functions
        if( !it ) return false;

        // If this isn't the first monitor to test this, there is no reason to repeat the test.
        if( this != group[0] ) return group[0]->mask.accepted >= 0;

        // For all acceptable functions check if this is the current function.
        for( short i = 0; i < count; i++, it++ ) {
                if( *it == group ) {
                        *this->mask.accepted = i;
                        return true;
                }
        }

        // No function matched
        return false;
}

static inline void init( int count, monitor_desc ** monitors, __condition_node_t * waiter, __condition_criterion_t * criteria ) {
        for(int i = 0; i < count; i++) {
                (criteria[i]){ monitors[i], waiter };
        }

        waiter->criteria = criteria;
}

static inline void init_push( int count, monitor_desc ** monitors, __condition_node_t * waiter, __condition_criterion_t * criteria ) {
        for(int i = 0; i < count; i++) {
                (criteria[i]){ monitors[i], waiter };
                push( &criteria[i].target->signal_stack, &criteria[i] );
        }

        waiter->criteria = criteria;
}

static inline void lock_all( spinlock ** locks, unsigned short count ) {
        for( int i = 0; i < count; i++ ) {
                lock_yield( locks[i] DEBUG_CTX2 );
        }
}

static inline void lock_all( monitor_desc ** source, spinlock ** /*out*/ locks, unsigned short count ) {
        for( int i = 0; i < count; i++ ) {
                spinlock * l = &source[i]->lock;
                lock_yield( l DEBUG_CTX2 );
                if(locks) locks[i] = l;
        }
}

static inline void unlock_all( spinlock ** locks, unsigned short count ) {
        for( int i = 0; i < count; i++ ) {
                unlock( locks[i] );
        }
}

static inline void unlock_all( monitor_desc ** locks, unsigned short count ) {
        for( int i = 0; i < count; i++ ) {
                unlock( &locks[i]->lock );
        }
}

static inline void save( monitor_desc ** ctx, short count, __attribute((unused)) spinlock ** locks, unsigned int * /*out*/ recursions, __waitfor_mask_t * /*out*/ masks ) {
        for( int i = 0; i < count; i++ ) {
                recursions[i] = ctx[i]->recursion;
                masks[i]      = ctx[i]->mask;
        }
}

static inline void restore( monitor_desc ** ctx, short count, spinlock ** locks, unsigned int * /*out*/ recursions, __waitfor_mask_t * /*out*/ masks ) {
        lock_all( locks, count );
        for( int i = 0; i < count; i++ ) {
                ctx[i]->recursion = recursions[i];
                ctx[i]->mask      = masks[i];
        }
        unlock_all( locks, count );
}

// Function has 2 different behavior
// 1 - Marks a monitors as being ready to run
// 2 - Checks if all the monitors are ready to run
//     if so return the thread to run
static inline thread_desc * check_condition( __condition_criterion_t * target ) {
        __condition_node_t * node = target->owner;
        unsigned short count = node->count;
        __condition_criterion_t * criteria = node->criteria;

        bool ready2run = true;

        for(    int i = 0; i < count; i++ ) {

                // LIB_DEBUG_PRINT_SAFE( "Checking %p for %p\n", &criteria[i], target );
                if( &criteria[i] == target ) {
                        criteria[i].ready = true;
                        // LIB_DEBUG_PRINT_SAFE( "True\n" );
                }

                ready2run = criteria[i].ready && ready2run;
        }

        // LIB_DEBUG_PRINT_SAFE( "Runing %i\n", ready2run );
        return ready2run ? node->waiting_thread : NULL;
}

static inline void brand_condition( condition * this ) {
        thread_desc * thrd = this_thread;
        if( !this->monitors ) {
                // LIB_DEBUG_PRINT_SAFE("Branding\n");
                assertf( thrd->monitors.list != NULL, "No current monitor to brand condition %p", thrd->monitors.list );
                this->monitor_count = thrd->monitors.size;

                this->monitors = malloc( this->monitor_count * sizeof( *this->monitors ) );
                for( int i = 0; i < this->monitor_count; i++ ) {
                        this->monitors[i] = thrd->monitors.list[i];
                }
        }
}

static inline [thread_desc *, int] search_entry_queue( const __waitfor_mask_t & mask, monitor_desc ** monitors, int count ) {

        __thread_queue_t * entry_queue = &monitors[0]->entry_queue;

        // For each thread in the entry-queue
        for(    thread_desc ** thrd_it = &entry_queue->head;
                *thrd_it;
                thrd_it = &(*thrd_it)->next
        ) {
                // For each acceptable check if it matches
                int i = 0;
                __acceptable_t * end = mask.clauses + mask.size;
                for( __acceptable_t * it = mask.clauses; it != end; it++, i++ ) {
                        // Check if we have a match
                        if( *it == (*thrd_it)->monitors ) {

                                // If we have a match return it
                                // after removeing it from the entry queue
                                return [remove( entry_queue, thrd_it ), i];
                        }
                }
        }

        return [0, -1];
}

forall(dtype T | sized( T ))
static inline short insert_unique( T ** array, short & size, T * val ) {
        if( !val ) return size;

        for(int i = 0; i <= size; i++) {
                if( array[i] == val ) return size;
        }

        array[size] = val;
        size = size + 1;
        return size;
}

static inline short count_max( const __waitfor_mask_t & mask ) {
        short max = 0;
        for( int i = 0; i < mask.size; i++ ) {
                max += mask.clauses[i].size;
        }
        return max;
}

static inline short aggregate( monitor_desc ** storage, const __waitfor_mask_t & mask ) {
        short size = 0;
        for( int i = 0; i < mask.size; i++ ) {
                for( int j = 0; j < mask.clauses[i].size; j++) {
                        insert_unique( storage, size, mask.clauses[i].list[j] );
                }
        }
        qsort( storage, size );
        return size;
}

void ?{}( __condition_blocked_queue_t & this ) {
        this.head = NULL;
        this.tail = &this.head;
}

void append( __condition_blocked_queue_t * this, __condition_node_t * c ) {
        verify(this->tail != NULL);
        *this->tail = c;
        this->tail = &c->next;
}

__condition_node_t * pop_head( __condition_blocked_queue_t * this ) {
        __condition_node_t * head = this->head;
        if( head ) {
                this->head = head->next;
                if( !head->next ) {
                        this->tail = &this->head;
                }
                head->next = NULL;
        }
        return head;
}

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