HelenOS sources

root/uspace/lib/c/generic/thread/fibril_synch.c

DEFINITIONS

1. fibril_rmutex_initialize
2. fibril_rmutex_destroy
3. fibril_rmutex_lock
4. fibril_rmutex_trylock
5. fibril_rmutex_unlock
6. __fibril_synch_init
7. __fibril_synch_fini
8. print_deadlock
9. check_fibril_for_deadlock
10. check_for_deadlock
11. fibril_mutex_lock
12. fibril_mutex_trylock
13. _fibril_mutex_unlock_unsafe
14. fibril_mutex_unlock
15. fibril_mutex_is_locked
16. fibril_rwlock_initialize
17. fibril_rwlock_read_lock
18. fibril_rwlock_write_lock
19. _fibril_rwlock_common_unlock
20. fibril_rwlock_read_unlock
21. fibril_rwlock_write_unlock
22. fibril_rwlock_is_read_locked
23. fibril_rwlock_is_write_locked
24. fibril_rwlock_is_locked
25. fibril_condvar_initialize
26. fibril_condvar_wait_timeout
27. fibril_condvar_wait
28. fibril_condvar_signal
29. fibril_condvar_broadcast
30. fibril_timer_func
31. fibril_timer_create
32. fibril_timer_destroy
33. fibril_timer_set
34. fibril_timer_set_locked
35. fibril_timer_clear
36. fibril_timer_clear_locked
37. fibril_semaphore_initialize
38. fibril_semaphore_up
39. fibril_semaphore_down
40. fibril_semaphore_down_timeout
41. fibril_semaphore_close

/*
 * Copyright (c) 2025 Jiri Svoboda
 * Copyright (c) 2009 Jakub Jermar
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/** @addtogroup libc
 * @{
 */
/** @file
 */

#include <fibril_synch.h>
#include <fibril.h>
#include <async.h>
#include <adt/list.h>
#include <time.h>
#include <errno.h>
#include <assert.h>
#include <stacktrace.h>
#include <stdlib.h>
#include <stdio.h>
#include <io/kio.h>
#include <mem.h>
#include <context.h>

#include "../private/async.h"
#include "../private/fibril.h"
#include "../private/futex.h"

errno_t fibril_rmutex_initialize(fibril_rmutex_t *m)
{
        return futex_initialize(&m->futex, 1);
}

void fibril_rmutex_destroy(fibril_rmutex_t *m)
{
        futex_destroy(&m->futex);
}

/**
 * Lock restricted mutex.
 * When a restricted mutex is locked, the fibril may not sleep or create new
 * threads. Any attempt to do so will abort the program.
 */
void fibril_rmutex_lock(fibril_rmutex_t *m)
{
        futex_lock(&m->futex);
        fibril_self()->rmutex_locks++;
}

bool fibril_rmutex_trylock(fibril_rmutex_t *m)
{
        if (futex_trylock(&m->futex)) {
                fibril_self()->rmutex_locks++;
                return true;
        } else {
                return false;
        }
}

void fibril_rmutex_unlock(fibril_rmutex_t *m)
{
        fibril_self()->rmutex_locks--;
        futex_unlock(&m->futex);
}

static fibril_local bool deadlocked = false;

static futex_t fibril_synch_futex;

void __fibril_synch_init(void)
{
        if (futex_initialize(&fibril_synch_futex, 1) != EOK)
                abort();
}

void __fibril_synch_fini(void)
{
        futex_destroy(&fibril_synch_futex);
}

typedef struct {
        link_t link;
        fibril_event_t event;
        fibril_mutex_t *mutex;
        fid_t fid;
} awaiter_t;

#define AWAITER_INIT { .fid = fibril_get_id() }

/** Print deadlock message nad blocking chain.
 *
 * @param oi Owner info for the resource being acquired
 * @param f Fibril that is trying to acquire the resource
 */
static void print_deadlock(fibril_owner_info_t *oi, fibril_t *f)
{
        // FIXME: Print to stderr.

        if (deadlocked) {
                kio_printf("Deadlock detected while printing deadlock. Aborting.\n");
                abort();
        }
        deadlocked = true;

        printf("Deadlock detected.\n");
        stacktrace_print();

        printf("Fibril %p waits for primitive %p.\n", f, oi);

        while (oi && oi->owned_by) {
                printf("Primitive %p is owned by fibril %p.\n",
                    oi, oi->owned_by);
                if (oi->owned_by == f)
                        break;
                stacktrace_print_fp_pc(
                    context_get_fp(&oi->owned_by->ctx),
                    context_get_pc(&oi->owned_by->ctx));
                printf("Fibril %p waits for primitive %p.\n",
                    oi->owned_by, oi->owned_by->waits_for);
                oi = oi->owned_by->waits_for;
        }
}

/** Check whether fibril trying to acquire a resource will cause deadlock.
 *
 * @param wanted_oi Owner info for the primitive that the fibril wants
 * @param fib Fibril that wants to aquire the primitive
 */
static void check_fibril_for_deadlock(fibril_owner_info_t *wanted_oi,
    fibril_t *fib)
{
        fibril_owner_info_t *oi;

        futex_assert_is_locked(&fibril_synch_futex);

        oi = wanted_oi;
        while (oi && oi->owned_by) {
                if (oi->owned_by == fib) {
                        futex_unlock(&fibril_synch_futex);
                        print_deadlock(wanted_oi, fib);
                        abort();
                }
                oi = oi->owned_by->waits_for;
        }
}

/** Check whether trying to acquire a resource will cause deadlock.
 *
 * @param oi Owner info for the primitive that the current fibril wants
 */
static void check_for_deadlock(fibril_owner_info_t *oi)
{
        check_fibril_for_deadlock(oi, fibril_self());
}

void fibril_mutex_lock(fibril_mutex_t *fm)
{
        fibril_t *f = (fibril_t *) fibril_get_id();

        futex_lock(&fibril_synch_futex);

        if (fm->counter-- > 0) {
                fm->oi.owned_by = f;
                futex_unlock(&fibril_synch_futex);
                return;
        }

        awaiter_t wdata = AWAITER_INIT;
        list_append(&wdata.link, &fm->waiters);
        check_for_deadlock(&fm->oi);
        f->waits_for = &fm->oi;

        futex_unlock(&fibril_synch_futex);

        fibril_wait_for(&wdata.event);
}

bool fibril_mutex_trylock(fibril_mutex_t *fm)
{
        bool locked = false;

        futex_lock(&fibril_synch_futex);
        if (fm->counter > 0) {
                fm->counter--;
                fm->oi.owned_by = (fibril_t *) fibril_get_id();
                locked = true;
        }
        futex_unlock(&fibril_synch_futex);

        return locked;
}

static void _fibril_mutex_unlock_unsafe(fibril_mutex_t *fm)
{
        assert(fm->oi.owned_by == (fibril_t *) fibril_get_id());

        if (fm->counter++ < 0) {
                awaiter_t *wdp = list_pop(&fm->waiters, awaiter_t, link);
                assert(wdp);

                fibril_t *f = (fibril_t *) wdp->fid;
                fm->oi.owned_by = f;
                f->waits_for = NULL;

                fibril_notify(&wdp->event);
        } else {
                fm->oi.owned_by = NULL;
        }
}

void fibril_mutex_unlock(fibril_mutex_t *fm)
{
        futex_lock(&fibril_synch_futex);
        _fibril_mutex_unlock_unsafe(fm);
        futex_unlock(&fibril_synch_futex);
}

bool fibril_mutex_is_locked(fibril_mutex_t *fm)
{
        futex_lock(&fibril_synch_futex);
        bool locked = (fm->oi.owned_by == (fibril_t *) fibril_get_id());
        futex_unlock(&fibril_synch_futex);
        return locked;
}

void fibril_rwlock_initialize(fibril_rwlock_t *frw)
{
        frw->oi.owned_by = NULL;
        frw->writers = 0;
        frw->readers = 0;
        list_initialize(&frw->waiters);
}

void fibril_rwlock_read_lock(fibril_rwlock_t *frw)
{
        fibril_t *f = (fibril_t *) fibril_get_id();

        futex_lock(&fibril_synch_futex);

        if (!frw->writers && list_empty(&frw->waiters)) {
                /* Consider the first reader the owner. */
                if (frw->readers++ == 0)
                        frw->oi.owned_by = f;
                futex_unlock(&fibril_synch_futex);
                return;
        }

        f->is_writer = false;

        awaiter_t wdata = AWAITER_INIT;
        list_append(&wdata.link, &frw->waiters);
        check_for_deadlock(&frw->oi);
        f->waits_for = &frw->oi;

        futex_unlock(&fibril_synch_futex);

        fibril_wait_for(&wdata.event);
}

void fibril_rwlock_write_lock(fibril_rwlock_t *frw)
{
        fibril_t *f = (fibril_t *) fibril_get_id();

        futex_lock(&fibril_synch_futex);

        if (!frw->writers && !frw->readers) {
                frw->oi.owned_by = f;
                frw->writers++;
                futex_unlock(&fibril_synch_futex);
                return;
        }

        f->is_writer = true;

        awaiter_t wdata = AWAITER_INIT;
        list_append(&wdata.link, &frw->waiters);
        check_for_deadlock(&frw->oi);
        f->waits_for = &frw->oi;

        futex_unlock(&fibril_synch_futex);

        fibril_wait_for(&wdata.event);
}

static void _fibril_rwlock_common_unlock(fibril_rwlock_t *frw)
{
        if (frw->readers) {
                if (--frw->readers) {
                        if (frw->oi.owned_by == (fibril_t *) fibril_get_id()) {
                                /*
                                 * If this reader fibril was considered the
                                 * owner of this rwlock, clear the ownership
                                 * information even if there are still more
                                 * readers.
                                 *
                                 * This is the limitation of the detection
                                 * mechanism rooted in the fact that tracking
                                 * all readers would require dynamically
                                 * allocated memory for keeping linkage info.
                                 */
                                frw->oi.owned_by = NULL;
                        }

                        return;
                }
        } else {
                frw->writers--;
        }

        assert(!frw->readers && !frw->writers);

        frw->oi.owned_by = NULL;

        while (!list_empty(&frw->waiters)) {
                link_t *tmp = list_first(&frw->waiters);
                awaiter_t *wdp;
                fibril_t *f;

                wdp = list_get_instance(tmp, awaiter_t, link);
                f = (fibril_t *) wdp->fid;

                if (f->is_writer) {
                        if (frw->readers)
                                break;
                        frw->writers++;
                } else {
                        frw->readers++;
                }

                f->waits_for = NULL;
                list_remove(&wdp->link);
                frw->oi.owned_by = f;
                fibril_notify(&wdp->event);

                if (frw->writers)
                        break;
        }
}

void fibril_rwlock_read_unlock(fibril_rwlock_t *frw)
{
        futex_lock(&fibril_synch_futex);
        assert(frw->readers > 0);
        _fibril_rwlock_common_unlock(frw);
        futex_unlock(&fibril_synch_futex);
}

void fibril_rwlock_write_unlock(fibril_rwlock_t *frw)
{
        futex_lock(&fibril_synch_futex);
        assert(frw->writers == 1);
        assert(frw->oi.owned_by == fibril_self());
        _fibril_rwlock_common_unlock(frw);
        futex_unlock(&fibril_synch_futex);
}

bool fibril_rwlock_is_read_locked(fibril_rwlock_t *frw)
{
        futex_lock(&fibril_synch_futex);
        bool locked = (frw->readers > 0);
        futex_unlock(&fibril_synch_futex);
        return locked;
}

bool fibril_rwlock_is_write_locked(fibril_rwlock_t *frw)
{
        futex_lock(&fibril_synch_futex);
        assert(frw->writers <= 1);
        bool locked = (frw->writers > 0) && (frw->oi.owned_by == fibril_self());
        futex_unlock(&fibril_synch_futex);
        return locked;
}

bool fibril_rwlock_is_locked(fibril_rwlock_t *frw)
{
        return fibril_rwlock_is_read_locked(frw) ||
            fibril_rwlock_is_write_locked(frw);
}

void fibril_condvar_initialize(fibril_condvar_t *fcv)
{
        list_initialize(&fcv->waiters);
}

/**
 * FIXME: If `timeout` is negative, the function returns ETIMEOUT immediately,
 *        and if `timeout` is 0, the wait never times out.
 *        This is not consistent with other similar APIs.
 */
errno_t
fibril_condvar_wait_timeout(fibril_condvar_t *fcv, fibril_mutex_t *fm,
    usec_t timeout)
{
        assert(fibril_mutex_is_locked(fm));

        if (timeout < 0)
                return ETIMEOUT;

        awaiter_t wdata = AWAITER_INIT;
        wdata.mutex = fm;

        struct timespec ts;
        struct timespec *expires = NULL;
        if (timeout) {
                getuptime(&ts);
                ts_add_diff(&ts, USEC2NSEC(timeout));
                expires = &ts;
        }

        futex_lock(&fibril_synch_futex);
        _fibril_mutex_unlock_unsafe(fm);
        list_append(&wdata.link, &fcv->waiters);
        futex_unlock(&fibril_synch_futex);

        (void) fibril_wait_timeout(&wdata.event, expires);

        futex_lock(&fibril_synch_futex);
        bool timed_out = link_in_use(&wdata.link);
        list_remove(&wdata.link);
        futex_unlock(&fibril_synch_futex);

        fibril_mutex_lock(fm);

        return timed_out ? ETIMEOUT : EOK;
}

void fibril_condvar_wait(fibril_condvar_t *fcv, fibril_mutex_t *fm)
{
        (void) fibril_condvar_wait_timeout(fcv, fm, 0);
}

void fibril_condvar_signal(fibril_condvar_t *fcv)
{
        futex_lock(&fibril_synch_futex);

        awaiter_t *w = list_pop(&fcv->waiters, awaiter_t, link);
        if (w != NULL)
                fibril_notify(&w->event);

        futex_unlock(&fibril_synch_futex);
}

void fibril_condvar_broadcast(fibril_condvar_t *fcv)
{
        futex_lock(&fibril_synch_futex);

        awaiter_t *w;
        while ((w = list_pop(&fcv->waiters, awaiter_t, link)))
                fibril_notify(&w->event);

        futex_unlock(&fibril_synch_futex);
}

/** Timer fibril.
 *
 * @param arg   Timer
 */
static errno_t fibril_timer_func(void *arg)
{
        fibril_timer_t *timer = (fibril_timer_t *) arg;
        errno_t rc;

        fibril_mutex_lock(timer->lockp);

        while (timer->state != fts_cleanup) {
                switch (timer->state) {
                case fts_not_set:
                case fts_fired:
                        fibril_condvar_wait(&timer->cv, timer->lockp);
                        break;
                case fts_active:
                        rc = fibril_condvar_wait_timeout(&timer->cv,
                            timer->lockp, timer->delay);
                        if (rc == ETIMEOUT && timer->state == fts_active) {
                                timer->state = fts_fired;
                                timer->handler_fid = fibril_get_id();
                                fibril_mutex_unlock(timer->lockp);
                                timer->fun(timer->arg);
                                fibril_mutex_lock(timer->lockp);
                                timer->handler_fid = 0;
                        }
                        break;
                case fts_cleanup:
                case fts_clean:
                        assert(false);
                        break;
                }
        }

        /* Acknowledge timer fibril has finished cleanup. */
        timer->state = fts_clean;
        fibril_condvar_broadcast(&timer->cv);
        fibril_mutex_unlock(timer->lockp);

        return 0;
}

/** Create new timer.
 *
 * @return              New timer on success, @c NULL if out of memory.
 */
fibril_timer_t *fibril_timer_create(fibril_mutex_t *lock)
{
        fid_t fid;
        fibril_timer_t *timer;

        timer = calloc(1, sizeof(fibril_timer_t));
        if (timer == NULL)
                return NULL;

        fid = fibril_create(fibril_timer_func, (void *) timer);
        if (fid == 0) {
                free(timer);
                return NULL;
        }

        fibril_mutex_initialize(&timer->lock);
        fibril_condvar_initialize(&timer->cv);

        timer->fibril = fid;
        timer->state = fts_not_set;
        timer->lockp = (lock != NULL) ? lock : &timer->lock;

        fibril_add_ready(fid);
        return timer;
}

/** Destroy timer.
 *
 * @param timer         Timer, must not be active or accessed by other threads.
 */
void fibril_timer_destroy(fibril_timer_t *timer)
{
        fibril_mutex_lock(timer->lockp);
        assert(timer->state == fts_not_set || timer->state == fts_fired);

        /* Request timer fibril to terminate. */
        timer->state = fts_cleanup;
        fibril_condvar_broadcast(&timer->cv);

        /* Wait for timer fibril to terminate */
        while (timer->state != fts_clean)
                fibril_condvar_wait(&timer->cv, timer->lockp);
        fibril_mutex_unlock(timer->lockp);

        free(timer);
}

/** Set timer.
 *
 * Set timer to execute a callback function after the specified
 * interval.
 *
 * @param timer         Timer
 * @param delay         Delay in microseconds
 * @param fun           Callback function
 * @param arg           Argument for @a fun
 */
void fibril_timer_set(fibril_timer_t *timer, usec_t delay,
    fibril_timer_fun_t fun, void *arg)
{
        fibril_mutex_lock(timer->lockp);
        fibril_timer_set_locked(timer, delay, fun, arg);
        fibril_mutex_unlock(timer->lockp);
}

/** Set locked timer.
 *
 * Set timer to execute a callback function after the specified
 * interval. Must be called when the timer is locked.
 *
 * @param timer         Timer
 * @param delay         Delay in microseconds
 * @param fun           Callback function
 * @param arg           Argument for @a fun
 */
void fibril_timer_set_locked(fibril_timer_t *timer, usec_t delay,
    fibril_timer_fun_t fun, void *arg)
{
        assert(fibril_mutex_is_locked(timer->lockp));
        assert(timer->state == fts_not_set || timer->state == fts_fired);
        timer->state = fts_active;
        timer->delay = delay;
        timer->fun = fun;
        timer->arg = arg;
        fibril_condvar_broadcast(&timer->cv);
}

/** Clear timer.
 *
 * Clears (cancels) timer and returns last state of the timer.
 * This can be one of:
 *    - fts_not_set     If the timer has not been set or has been cleared
 *    - fts_active      Timer was set but did not fire
 *    - fts_fired       Timer fired
 *
 * @param timer         Timer
 * @return              Last timer state
 */
fibril_timer_state_t fibril_timer_clear(fibril_timer_t *timer)
{
        fibril_timer_state_t old_state;

        fibril_mutex_lock(timer->lockp);
        old_state = fibril_timer_clear_locked(timer);
        fibril_mutex_unlock(timer->lockp);

        return old_state;
}

/** Clear locked timer.
 *
 * Clears (cancels) timer and returns last state of the timer.
 * This can be one of:
 *    - fts_not_set     If the timer has not been set or has been cleared
 *    - fts_active      Timer was set but did not fire
 *    - fts_fired       Timer fired
 * Must be called when the timer is locked.
 *
 * @param timer         Timer
 * @return              Last timer state
 */
fibril_timer_state_t fibril_timer_clear_locked(fibril_timer_t *timer)
{
        fibril_timer_state_t old_state;

        assert(fibril_mutex_is_locked(timer->lockp));

        while (timer->handler_fid != 0) {
                if (timer->handler_fid == fibril_get_id()) {
                        printf("Deadlock detected.\n");
                        stacktrace_print();
                        printf("Fibril %p is trying to clear timer %p from "
                            "inside its handler %p.\n",
                            fibril_get_id(), timer, timer->fun);
                        abort();
                }

                fibril_condvar_wait(&timer->cv, timer->lockp);
        }

        old_state = timer->state;
        timer->state = fts_not_set;

        timer->delay = 0;
        timer->fun = NULL;
        timer->arg = NULL;
        fibril_condvar_broadcast(&timer->cv);

        return old_state;
}

/**
 * Initialize a semaphore with initial count set to the provided value.
 *
 * @param sem    Semaphore to initialize.
 * @param count  Initial count. Must not be negative.
 */
void fibril_semaphore_initialize(fibril_semaphore_t *sem, long count)
{
        /*
         * Negative count denotes the length of waitlist,
         * so it makes no sense as an initial value.
         */
        assert(count >= 0);
        sem->closed = false;
        sem->count = count;
        list_initialize(&sem->waiters);
}

/**
 * Produce one token.
 * If there are fibrils waiting for tokens, this operation satisfies
 * exactly one waiting `fibril_semaphore_down()`.
 * This operation never blocks the fibril.
 *
 * @param sem  Semaphore to use.
 */
void fibril_semaphore_up(fibril_semaphore_t *sem)
{
        futex_lock(&fibril_synch_futex);

        if (sem->closed) {
                futex_unlock(&fibril_synch_futex);
                return;
        }

        sem->count++;

        if (sem->count <= 0) {
                awaiter_t *w = list_pop(&sem->waiters, awaiter_t, link);
                assert(w);
                fibril_notify(&w->event);
        }

        futex_unlock(&fibril_synch_futex);
}

/**
 * Consume one token.
 * If there are no available tokens (count <= 0), this operation blocks until
 * another fibril produces a token using `fibril_semaphore_up()`.
 *
 * @param sem  Semaphore to use.
 */
void fibril_semaphore_down(fibril_semaphore_t *sem)
{
        futex_lock(&fibril_synch_futex);

        if (sem->closed) {
                futex_unlock(&fibril_synch_futex);
                return;
        }

        sem->count--;

        if (sem->count >= 0) {
                futex_unlock(&fibril_synch_futex);
                return;
        }

        awaiter_t wdata = AWAITER_INIT;
        list_append(&wdata.link, &sem->waiters);

        futex_unlock(&fibril_synch_futex);

        fibril_wait_for(&wdata.event);
}

errno_t fibril_semaphore_down_timeout(fibril_semaphore_t *sem, usec_t timeout)
{
        if (timeout < 0)
                return ETIMEOUT;

        futex_lock(&fibril_synch_futex);
        if (sem->closed) {
                futex_unlock(&fibril_synch_futex);
                return EOK;
        }

        sem->count--;

        if (sem->count >= 0) {
                futex_unlock(&fibril_synch_futex);
                return EOK;
        }

        awaiter_t wdata = AWAITER_INIT;
        list_append(&wdata.link, &sem->waiters);

        futex_unlock(&fibril_synch_futex);

        struct timespec ts;
        struct timespec *expires = NULL;
        if (timeout) {
                getuptime(&ts);
                ts_add_diff(&ts, USEC2NSEC(timeout));
                expires = &ts;
        }

        errno_t rc = fibril_wait_timeout(&wdata.event, expires);
        if (rc == EOK)
                return EOK;

        futex_lock(&fibril_synch_futex);
        if (!link_in_use(&wdata.link)) {
                futex_unlock(&fibril_synch_futex);
                return EOK;
        }

        list_remove(&wdata.link);
        sem->count++;
        futex_unlock(&fibril_synch_futex);

        return rc;
}

/**
 * Close the semaphore.
 * All future down() operations return instantly.
 */
void fibril_semaphore_close(fibril_semaphore_t *sem)
{
        futex_lock(&fibril_synch_futex);
        sem->closed = true;
        awaiter_t *w;

        while ((w = list_pop(&sem->waiters, awaiter_t, link)))
                fibril_notify(&w->event);

        futex_unlock(&fibril_synch_futex);
}

/** @}
 */