source: libcfa/src/concurrency/locks.hfa@ f3ba69d

//
// 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.
//
// locks.hfa -- PUBLIC
// Runtime locks that used with the runtime thread system.
//
// Author           : Colby Alexander Parsons
// Created On       : Thu Jan 21 19:46:50 2021
// Last Modified By :
// Last Modified On :
// Update Count     :
//

#pragma once

#include <stdbool.h>
#include <stdio.h>

#include "bits/weakso_locks.hfa"
#include "containers/lockfree.hfa"
#include "containers/list.hfa"

#include "limits.hfa"
#include "thread.hfa"

#include "time_t.hfa"
#include "time.hfa"

#include <fstream.hfa>

// futex headers
#include <linux/futex.h>      /* Definition of FUTEX_* constants */
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

// C_TODO: cleanup this and locks.cfa
// - appropriate separation of interface and impl
// - clean up unused/unneeded locks
// - change messy big blocking lock from inheritance to composition to remove need for flags

//-----------------------------------------------------------------------------
// Semaphore
struct semaphore {
        __spinlock_t lock;
        int count;
        __queue_t(thread$) waiting;
};

void  ?{}(semaphore & this, int count = 1);
void ^?{}(semaphore & this);
bool   P (semaphore & this);
bool   V (semaphore & this);
bool   V (semaphore & this, unsigned count);
thread$ * V (semaphore & this, bool );

//----------
struct single_acquisition_lock {
        inline blocking_lock;
};

static inline void  ?{}( single_acquisition_lock & this ) {((blocking_lock &)this){ false, false };}
static inline void ^?{}( single_acquisition_lock & this ) {}
static inline void   lock     ( single_acquisition_lock & this ) { lock    ( (blocking_lock &)this ); }
static inline bool   try_lock ( single_acquisition_lock & this ) { return try_lock( (blocking_lock &)this ); }
static inline void   unlock   ( single_acquisition_lock & this ) { unlock  ( (blocking_lock &)this ); }
static inline size_t on_wait  ( single_acquisition_lock & this ) { return on_wait ( (blocking_lock &)this ); }
static inline void   on_wakeup( single_acquisition_lock & this, size_t v ) { on_wakeup ( (blocking_lock &)this, v ); }
static inline void   on_notify( single_acquisition_lock & this, struct thread$ * t ) { on_notify( (blocking_lock &)this, t ); }

//----------
struct owner_lock {
        inline blocking_lock;
};

static inline void  ?{}( owner_lock & this ) {((blocking_lock &)this){ true, true };}
static inline void ^?{}( owner_lock & this ) {}
static inline void   lock     ( owner_lock & this ) { lock    ( (blocking_lock &)this ); }
static inline bool   try_lock ( owner_lock & this ) { return try_lock( (blocking_lock &)this ); }
static inline void   unlock   ( owner_lock & this ) { unlock  ( (blocking_lock &)this ); }
static inline size_t on_wait  ( owner_lock & this ) { return on_wait ( (blocking_lock &)this ); }
static inline void   on_wakeup( owner_lock & this, size_t v ) { on_wakeup ( (blocking_lock &)this, v ); }
static inline void   on_notify( owner_lock & this, struct thread$ * t ) { on_notify( (blocking_lock &)this, t ); }

//-----------------------------------------------------------------------------
// MCS Lock
struct mcs_node {
        mcs_node * volatile next;
        single_sem sem;
};

static inline void ?{}(mcs_node & this) { this.next = 0p; }

static inline mcs_node * volatile & ?`next ( mcs_node * node ) {
        return node->next;
}

struct mcs_lock {
        mcs_queue(mcs_node) queue;
};

static inline void lock(mcs_lock & l, mcs_node & n) {
        if(push(l.queue, &n))
                wait(n.sem);
}

static inline void unlock(mcs_lock & l, mcs_node & n) {
        mcs_node * next = advance(l.queue, &n);
        if(next) post(next->sem);
}

//-----------------------------------------------------------------------------
// MCS Spin Lock
// - No recursive acquisition
// - Needs to be released by owner

struct mcs_spin_node {
        mcs_spin_node * volatile next;
        volatile bool locked;
};

struct mcs_spin_queue {
        mcs_spin_node * volatile tail;
};

static inline void ?{}(mcs_spin_node & this) { this.next = 0p; this.locked = true; }

static inline mcs_spin_node * volatile & ?`next ( mcs_spin_node * node ) {
        return node->next;
}

struct mcs_spin_lock {
        mcs_spin_queue queue;
};

static inline void lock(mcs_spin_lock & l, mcs_spin_node & n) {
        mcs_spin_node * prev = __atomic_exchange_n(&l.queue.tail, &n, __ATOMIC_SEQ_CST);
        n.locked = true;
        if(prev == 0p) return;
        prev->next = &n;
        while(__atomic_load_n(&n.locked, __ATOMIC_RELAXED)) Pause();
}

static inline void unlock(mcs_spin_lock & l, mcs_spin_node & n) {
        mcs_spin_node * n_ptr = &n;
        if (__atomic_compare_exchange_n(&l.queue.tail, &n_ptr, 0p, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) return;
        while (__atomic_load_n(&n.next, __ATOMIC_RELAXED) == 0p) {}
        n.next->locked = false;
}

//-----------------------------------------------------------------------------
// futex_mutex

// - Kernel thd blocking alternative to the spinlock
// - No ownership (will deadlock on reacq)
struct futex_mutex {
        // lock state any state other than UNLOCKED is locked
        // enum LockState { UNLOCKED = 0, UNCONTENDED = 1, CONTENDED = 2 };
        
        // stores a lock state
        int val; 
};

// to use for FUTEX_WAKE and FUTEX_WAIT (other futex calls will need more params)
static inline int futex(int *uaddr, int futex_op, int val) {
    return syscall(SYS_futex, uaddr, futex_op, val, NULL, NULL, 0);
}

static inline void  ?{}( futex_mutex & this ) with(this) { val = 0; }

static inline bool internal_try_lock(futex_mutex & this, int & compare_val) with(this) {
        return __atomic_compare_exchange_n((int*)&val, (int*)&compare_val, 1, false, __ATOMIC_ACQUIRE, __ATOMIC_ACQUIRE);
}

static inline int internal_exchange(futex_mutex & this) with(this) {
        return __atomic_exchange_n((int*)&val, 2, __ATOMIC_ACQUIRE);
}

// if this is called recursively IT WILL DEADLOCK!!!!!
static inline void lock(futex_mutex & this) with(this) {
        int state;

        for( int spin = 4; spin < 1024; spin += spin) {
                state = 0;
                // if unlocked, lock and return
                if (internal_try_lock(this, state)) return;
                if (2 == state) break;
                for (int i = 0; i < spin; i++) Pause();
        }

        // // no contention try to acquire
        // if (internal_try_lock(this, state)) return;
        
        // if not in contended state, set to be in contended state
        if (state != 2) state = internal_exchange(this);

        // block and spin until we win the lock
        while (state != 0) {
                futex((int*)&val, FUTEX_WAIT, 2); // if val is not 2 this returns with EWOULDBLOCK
                state = internal_exchange(this);
        }
}

static inline void unlock(futex_mutex & this) with(this) {
        // if uncontended do atomic unlock and then return
    if (__atomic_exchange_n(&val, 0, __ATOMIC_RELEASE) == 1) return;
        
        // otherwise threads are blocked so we must wake one
        futex((int *)&val, FUTEX_WAKE, 1);
}

static inline void on_notify( futex_mutex & f, thread$ * t){ unpark(t); }
static inline size_t on_wait( futex_mutex & f ) {unlock(f); return 0;}

// to set recursion count after getting signalled;
static inline void on_wakeup( futex_mutex & f, size_t recursion ) {}

//-----------------------------------------------------------------------------
// go_mutex

// - Kernel thd blocking alternative to the spinlock
// - No ownership (will deadlock on reacq)
// - Golang's flavour of mutex
// - Impl taken from Golang: src/runtime/lock_futex.go
struct go_mutex {
        // lock state any state other than UNLOCKED is locked
        // enum LockState { UNLOCKED = 0, LOCKED = 1, SLEEPING = 2 };
        
        // stores a lock state
        int val; 
};

static inline void  ?{}( go_mutex & this ) with(this) { val = 0; }

static inline bool internal_try_lock(go_mutex & this, int & compare_val, int new_val ) with(this) {
        return __atomic_compare_exchange_n((int*)&val, (int*)&compare_val, new_val, false, __ATOMIC_ACQUIRE, __ATOMIC_ACQUIRE);
}

static inline int internal_exchange(go_mutex & this, int swap ) with(this) {
        return __atomic_exchange_n((int*)&val, swap, __ATOMIC_ACQUIRE);
}

// if this is called recursively IT WILL DEADLOCK!!!!!
static inline void lock(go_mutex & this) with(this) {
        int state, init_state;

    // speculative grab
    state = internal_exchange(this, 1);
    if ( !state ) return; // state == 0
    init_state = state;
    for (;;) {
        for( int i = 0; i < 4; i++ ) {
            while( !val ) { // lock unlocked
                state = 0;
                if (internal_try_lock(this, state, init_state)) return;
            }
            for (int i = 0; i < 30; i++) Pause();
        }

        while( !val ) { // lock unlocked
            state = 0;
            if (internal_try_lock(this, state, init_state)) return;
        }
        sched_yield();
        
        // if not in contended state, set to be in contended state
        state = internal_exchange(this, 2);
        if ( !state ) return; // state == 0
        init_state = 2;
        futex((int*)&val, FUTEX_WAIT, 2); // if val is not 2 this returns with EWOULDBLOCK
    }
}

static inline void unlock( go_mutex & this ) with(this) {
        // if uncontended do atomic unlock and then return
    if (__atomic_exchange_n(&val, 0, __ATOMIC_RELEASE) == 1) return;
        
        // otherwise threads are blocked so we must wake one
        futex((int *)&val, FUTEX_WAKE, 1);
}

static inline void on_notify( go_mutex & f, thread$ * t){ unpark(t); }
static inline size_t on_wait( go_mutex & f ) {unlock(f); return 0;}
static inline void on_wakeup( go_mutex & f, size_t recursion ) {}

//-----------------------------------------------------------------------------
// CLH Spinlock
// - No recursive acquisition
// - Needs to be released by owner

struct clh_lock {
        volatile bool * volatile tail;
    volatile bool * volatile head;
};

static inline void  ?{}( clh_lock & this ) { this.tail = malloc(); *this.tail = true; }
static inline void ^?{}( clh_lock & this ) { free(this.tail); }

static inline void lock(clh_lock & l) {
        thread$ * curr_thd = active_thread();
        *(curr_thd->clh_node) = false;
        volatile bool * prev = __atomic_exchange_n((bool **)(&l.tail), (bool *)(curr_thd->clh_node), __ATOMIC_SEQ_CST);
        while(!__atomic_load_n(prev, __ATOMIC_SEQ_CST)) Pause();
    __atomic_store_n((bool **)(&l.head), (bool *)curr_thd->clh_node, __ATOMIC_SEQ_CST);
    curr_thd->clh_node = prev;
}

static inline void unlock(clh_lock & l) {
        __atomic_store_n((bool *)(l.head), true, __ATOMIC_SEQ_CST);
}

static inline void on_notify(clh_lock & this, struct thread$ * t ) { unpark(t); }
static inline size_t on_wait(clh_lock & this) { unlock(this); return 0; }
static inline void on_wakeup(clh_lock & this, size_t recursion ) { lock(this); }

//-----------------------------------------------------------------------------
// Exponential backoff then block lock
struct exp_backoff_then_block_lock {
        // Spin lock used for mutual exclusion
        __spinlock_t spinlock;

        // List of blocked threads
        dlist( thread$ ) blocked_threads;

        // Used for comparing and exchanging
        volatile size_t lock_value;
};

static inline void  ?{}( exp_backoff_then_block_lock & this ) {
        this.spinlock{};
        this.blocked_threads{};
        this.lock_value = 0;
}

static inline void  ^?{}( exp_backoff_then_block_lock & this ){}

static inline bool internal_try_lock(exp_backoff_then_block_lock & this, size_t & compare_val) with(this) {
        return __atomic_compare_exchange_n(&lock_value, &compare_val, 1, false, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
}

static inline bool try_lock(exp_backoff_then_block_lock & this) { size_t compare_val = 0; return internal_try_lock(this, compare_val); }

static inline bool try_lock_contention(exp_backoff_then_block_lock & this) with(this) {
        return !__atomic_exchange_n(&lock_value, 2, __ATOMIC_ACQUIRE);
}

static inline bool block(exp_backoff_then_block_lock & this) with(this) {
    lock( spinlock __cfaabi_dbg_ctx2 );
    if (__atomic_load_n( &lock_value, __ATOMIC_SEQ_CST) != 2) {
        unlock( spinlock );
        return true;
    }
    insert_last( blocked_threads, *active_thread() );
    unlock( spinlock );
        park( );
        return true;
}

static inline void lock(exp_backoff_then_block_lock & this) with(this) {
        size_t compare_val = 0;
        int spin = 4;

        // linear backoff
        for( ;; ) {
                compare_val = 0;
                if (internal_try_lock(this, compare_val)) return;
                if (2 == compare_val) break;
                for (int i = 0; i < spin; i++) Pause();
                if (spin >= 1024) break;
                spin += spin;
        }

        if(2 != compare_val && try_lock_contention(this)) return;
        // block until signalled
        while (block(this)) if(try_lock_contention(this)) return;
}

static inline void unlock(exp_backoff_then_block_lock & this) with(this) {
    if (__atomic_exchange_n(&lock_value, 0, __ATOMIC_RELEASE) == 1) return;
    lock( spinlock __cfaabi_dbg_ctx2 );
    thread$ * t = &try_pop_front( blocked_threads );
    unlock( spinlock );
    unpark( t );
}

static inline void on_notify(exp_backoff_then_block_lock & this, struct thread$ * t ) { unpark(t); }
static inline size_t on_wait(exp_backoff_then_block_lock & this) { unlock(this); return 0; }
static inline void on_wakeup(exp_backoff_then_block_lock & this, size_t recursion ) { lock(this); }

//-----------------------------------------------------------------------------
// Fast Block Lock

// minimal blocking lock
// - No reacquire for cond var
// - No recursive acquisition
// - No ownership
struct fast_block_lock {
        // List of blocked threads
        dlist( thread$ ) blocked_threads;

        // Spin lock used for mutual exclusion
        __spinlock_t lock;

        // flag showing if lock is held
        bool held:1;
};

static inline void  ?{}( fast_block_lock & this ) with(this) {
        lock{};
        blocked_threads{};
        held = false;
}
static inline void ^?{}( fast_block_lock & this ) {}
static inline void ?{}( fast_block_lock & this, fast_block_lock this2 ) = void;
static inline void ?=?( fast_block_lock & this, fast_block_lock this2 ) = void;

// if this is called recursively IT WILL DEADLOCK!!!!!
static inline void lock(fast_block_lock & this) with(this) {
        lock( lock __cfaabi_dbg_ctx2 );
        if ( held ) {
                insert_last( blocked_threads, *active_thread() );
                unlock( lock );
                park( );
                return;
        }
        held = true;
        unlock( lock );
}

static inline void unlock(fast_block_lock & this) with(this) {
        lock( lock __cfaabi_dbg_ctx2 );
        /* paranoid */ verifyf( held != false, "Attempt to release lock %p that isn't held", &this );
        thread$ * t = &try_pop_front( blocked_threads );
        held = ( t ? true : false );
        unpark( t );
        unlock( lock );
}

static inline void on_notify(fast_block_lock & this, struct thread$ * t ) with(this) {
    lock( lock __cfaabi_dbg_ctx2 );
    insert_last( blocked_threads, *t );
    unlock( lock );
}
static inline size_t on_wait(fast_block_lock & this) { unlock(this); return 0; }
static inline void on_wakeup(fast_block_lock & this, size_t recursion ) { }

//-----------------------------------------------------------------------------
// simple_owner_lock

// pthread owner lock
// - reacquire for cond var
// - recursive acquisition
// - ownership
struct simple_owner_lock {
        // List of blocked threads
        dlist( thread$ ) blocked_threads;

        // Spin lock used for mutual exclusion
        __spinlock_t lock;

        // owner showing if lock is held
        struct thread$ * owner;

        size_t recursion_count;
};

static inline void  ?{}( simple_owner_lock & this ) with(this) {
        lock{};
        blocked_threads{};
        owner = 0p;
        recursion_count = 0;
}
static inline void ^?{}( simple_owner_lock & this ) {}
static inline void ?{}( simple_owner_lock & this, simple_owner_lock this2 ) = void;
static inline void ?=?( simple_owner_lock & this, simple_owner_lock this2 ) = void;

static inline void lock(simple_owner_lock & this) with(this) {
        if (owner == active_thread()) {
                recursion_count++;
                return;
        }
        lock( lock __cfaabi_dbg_ctx2 );

        if (owner != 0p) {
                insert_last( blocked_threads, *active_thread() );
                unlock( lock );
                park( );
                return;
        }
        owner = active_thread();
        recursion_count = 1;
        unlock( lock );
}

// TODO: fix duplicate def issue and bring this back
// void pop_and_set_new_owner( simple_owner_lock & this ) with( this ) {
        // thread$ * t = &try_pop_front( blocked_threads );
        // owner = t;
        // recursion_count = ( t ? 1 : 0 );
        // unpark( t );
// }

static inline void unlock(simple_owner_lock & this) with(this) {
        lock( lock __cfaabi_dbg_ctx2 );
        /* paranoid */ verifyf( owner != 0p, "Attempt to release lock %p that isn't held", &this );
        /* paranoid */ verifyf( owner == active_thread(), "Thread %p other than the owner %p attempted to release owner lock %p", owner, active_thread(), &this );
        // if recursion count is zero release lock and set new owner if one is waiting
        recursion_count--;
        if ( recursion_count == 0 ) {
                // pop_and_set_new_owner( this );
                thread$ * t = &try_pop_front( blocked_threads );
                owner = t;
                recursion_count = ( t ? 1 : 0 );
                unpark( t );
        }
        unlock( lock );
}

static inline void on_notify(simple_owner_lock & this, struct thread$ * t ) with(this) {
        lock( lock __cfaabi_dbg_ctx2 );
        // lock held
        if ( owner != 0p ) {
                insert_last( blocked_threads, *t );
        }
        // lock not held
        else {
                owner = t;
                recursion_count = 1;
                unpark( t );
        }
        unlock( lock );
}

static inline size_t on_wait(simple_owner_lock & this) with(this) {
        lock( lock __cfaabi_dbg_ctx2 );
        /* paranoid */ verifyf( owner != 0p, "Attempt to release lock %p that isn't held", &this );
        /* paranoid */ verifyf( owner == active_thread(), "Thread %p other than the owner %p attempted to release owner lock %p", owner, active_thread(), &this );

        size_t ret = recursion_count;

        // pop_and_set_new_owner( this );

        thread$ * t = &try_pop_front( blocked_threads );
        owner = t;
        recursion_count = ( t ? 1 : 0 );
        unpark( t );

        unlock( lock );
        return ret;
}

static inline void on_wakeup(simple_owner_lock & this, size_t recursion ) with(this) { recursion_count = recursion; }

//-----------------------------------------------------------------------------
// Spin Queue Lock

// - No reacquire for cond var
// - No recursive acquisition
// - No ownership
// - spin lock with no locking/atomics in unlock
struct spin_queue_lock {
        // Spin lock used for mutual exclusion
        mcs_spin_lock lock;

        // flag showing if lock is held
        volatile bool held;
};

static inline void  ?{}( spin_queue_lock & this ) with(this) {
        lock{};
        held = false;
}
static inline void ^?{}( spin_queue_lock & this ) {}
static inline void ?{}( spin_queue_lock & this, spin_queue_lock this2 ) = void;
static inline void ?=?( spin_queue_lock & this, spin_queue_lock this2 ) = void;

// if this is called recursively IT WILL DEADLOCK!
static inline void lock(spin_queue_lock & this) with(this) {
        mcs_spin_node node;
        lock( lock, node );
        while(__atomic_load_n(&held, __ATOMIC_SEQ_CST)) Pause();
        __atomic_store_n(&held, true, __ATOMIC_SEQ_CST);
        unlock( lock, node );
}

static inline void unlock(spin_queue_lock & this) with(this) {
        __atomic_store_n(&held, false, __ATOMIC_RELEASE);
}

static inline void on_notify(spin_queue_lock & this, struct thread$ * t ) {
        unpark(t);
}
static inline size_t on_wait(spin_queue_lock & this) { unlock(this); return 0; }
static inline void on_wakeup(spin_queue_lock & this, size_t recursion ) { lock(this); }


//-----------------------------------------------------------------------------
// MCS Block Spin Lock

// - No reacquire for cond var
// - No recursive acquisition
// - No ownership
// - Blocks but first node spins (like spin queue but blocking for not first thd)
struct mcs_block_spin_lock {
        // Spin lock used for mutual exclusion
        mcs_lock lock;

        // flag showing if lock is held
        volatile bool held;
};

static inline void  ?{}( mcs_block_spin_lock & this ) with(this) {
        lock{};
        held = false;
}
static inline void ^?{}( mcs_block_spin_lock & this ) {}
static inline void ?{}( mcs_block_spin_lock & this, mcs_block_spin_lock this2 ) = void;
static inline void ?=?( mcs_block_spin_lock & this, mcs_block_spin_lock this2 ) = void;

// if this is called recursively IT WILL DEADLOCK!!!!!
static inline void lock(mcs_block_spin_lock & this) with(this) {
        mcs_node node;
        lock( lock, node );
        while(__atomic_load_n(&held, __ATOMIC_SEQ_CST)) Pause();
        __atomic_store_n(&held, true, __ATOMIC_SEQ_CST);
        unlock( lock, node );
}

static inline void unlock(mcs_block_spin_lock & this) with(this) {
        __atomic_store_n(&held, false, __ATOMIC_SEQ_CST);
}

static inline void on_notify(mcs_block_spin_lock & this, struct thread$ * t ) { unpark(t); }
static inline size_t on_wait(mcs_block_spin_lock & this) { unlock(this); return 0; }
static inline void on_wakeup(mcs_block_spin_lock & this, size_t recursion ) {lock(this); }

//-----------------------------------------------------------------------------
// Block Spin Lock

// - No reacquire for cond var
// - No recursive acquisition
// - No ownership
// - Blocks but first node spins (like spin queue but blocking for not first thd)
struct block_spin_lock {
        // Spin lock used for mutual exclusion
        fast_block_lock lock;

        // flag showing if lock is held
        volatile bool held;
};

static inline void  ?{}( block_spin_lock & this ) with(this) {
        lock{};
        held = false;
}
static inline void ^?{}( block_spin_lock & this ) {}
static inline void ?{}( block_spin_lock & this, block_spin_lock this2 ) = void;
static inline void ?=?( block_spin_lock & this, block_spin_lock this2 ) = void;

// if this is called recursively IT WILL DEADLOCK!!!!!
static inline void lock(block_spin_lock & this) with(this) {
        lock( lock );
        while(__atomic_load_n(&held, __ATOMIC_SEQ_CST)) Pause();
        __atomic_store_n(&held, true, __ATOMIC_RELEASE);
        unlock( lock );
}

static inline void unlock(block_spin_lock & this) with(this) {
        __atomic_store_n(&held, false, __ATOMIC_RELEASE);
}

static inline void on_notify(block_spin_lock & this, struct thread$ * t ) with(this.lock) {
        // first we acquire internal fast_block_lock
        lock( lock __cfaabi_dbg_ctx2 );
        if ( held ) { // if internal fast_block_lock is held
                insert_last( blocked_threads, *t );
                unlock( lock );
                return;
        }
        // if internal fast_block_lock is not held
        held = true;
        unlock( lock );

        unpark(t);
}
static inline size_t on_wait(block_spin_lock & this) { unlock(this); return 0; }
static inline void on_wakeup(block_spin_lock & this, size_t recursion ) with(this) {
        // now we acquire the entire block_spin_lock upon waking up
        while(__atomic_load_n(&held, __ATOMIC_SEQ_CST)) Pause();
        __atomic_store_n(&held, true, __ATOMIC_RELEASE);
        unlock( lock ); // Now we release the internal fast_spin_lock
}

//-----------------------------------------------------------------------------
// is_blocking_lock
forall( L & | sized(L) )
trait is_blocking_lock {
        // For synchronization locks to use when acquiring
        void on_notify( L &, struct thread$ * );

        // For synchronization locks to use when releasing
        size_t on_wait( L & );

        // to set recursion count after getting signalled;
        void on_wakeup( L &, size_t recursion );
};

//-----------------------------------------------------------------------------
// // info_thread
// // the info thread is a wrapper around a thread used
// // to store extra data for use in the condition variable
forall(L & | is_blocking_lock(L)) {
        struct info_thread;

        // // for use by sequence
        // info_thread(L) *& Back( info_thread(L) * this );
        // info_thread(L) *& Next( info_thread(L) * this );
}

//-----------------------------------------------------------------------------
// Synchronization Locks
forall(L & | is_blocking_lock(L)) {

        //-----------------------------------------------------------------------------
        // condition_variable

        // The multi-tool condition variable
        // - can pass timeouts to wait for either a signal or timeout
        // - can wait without passing a lock
        // - can have waiters reacquire different locks while waiting on the same cond var
        // - has shadow queue
        // - can be signalled outside of critical sections with no locks held
        struct condition_variable {
                // Spin lock used for mutual exclusion
                __spinlock_t lock;

                // List of blocked threads
                dlist( info_thread(L) ) blocked_threads;

                // Count of current blocked threads
                int count;
        };


        void  ?{}( condition_variable(L) & this );
        void ^?{}( condition_variable(L) & this );

        bool notify_one( condition_variable(L) & this );
        bool notify_all( condition_variable(L) & this );

        uintptr_t front( condition_variable(L) & this );

        bool empty  ( condition_variable(L) & this );
        int  counter( condition_variable(L) & this );

        void wait( condition_variable(L) & this );
        void wait( condition_variable(L) & this, uintptr_t info );
        bool wait( condition_variable(L) & this, Duration duration );
        bool wait( condition_variable(L) & this, uintptr_t info, Duration duration );

        void wait( condition_variable(L) & this, L & l );
        void wait( condition_variable(L) & this, L & l, uintptr_t info );
        bool wait( condition_variable(L) & this, L & l, Duration duration );
        bool wait( condition_variable(L) & this, L & l, uintptr_t info, Duration duration );

        //-----------------------------------------------------------------------------
        // fast_cond_var

        // The trimmed and slim condition variable
        // - no internal lock so you must hold a lock while using this cond var
        // - signalling without holding branded lock is UNSAFE!
        // - only allows usage of one lock, cond var is branded after usage

        struct fast_cond_var {
                // List of blocked threads
                dlist( info_thread(L) ) blocked_threads;
                #ifdef __CFA_DEBUG__
                L * lock_used;
                #endif
        };

        void  ?{}( fast_cond_var(L) & this );
        void ^?{}( fast_cond_var(L) & this );

        bool notify_one( fast_cond_var(L) & this );
        bool notify_all( fast_cond_var(L) & this );

        uintptr_t front( fast_cond_var(L) & this );
        bool empty  ( fast_cond_var(L) & this );

        void wait( fast_cond_var(L) & this, L & l );
        void wait( fast_cond_var(L) & this, L & l, uintptr_t info );


        //-----------------------------------------------------------------------------
        // pthread_cond_var
        //
        // - cond var with minimal footprint
        // - supports operations needed for phthread cond

        struct pthread_cond_var {
                dlist( info_thread(L) ) blocked_threads;
                __spinlock_t lock;
        };

        void  ?{}( pthread_cond_var(L) & this );
        void ^?{}( pthread_cond_var(L) & this );

        bool notify_one( pthread_cond_var(L) & this );
        bool notify_all( pthread_cond_var(L) & this );

        uintptr_t front( pthread_cond_var(L) & this );
        bool empty ( pthread_cond_var(L) & this );

        void wait( pthread_cond_var(L) & this, L & l );
        void wait( pthread_cond_var(L) & this, L & l, uintptr_t info );
        bool wait( pthread_cond_var(L) & this, L & l, timespec t );
        bool wait( pthread_cond_var(L) & this, L & l, uintptr_t info, timespec t );
}