libsel4sync/include/sync/bin_sem_bare.h

/*
 * Copyright 2017, Data61
 * Commonwealth Scientific and Industrial Research Organisation (CSIRO)
 * ABN 41 687 119 230.
 *
 * This software may be distributed and modified according to the terms of
 * the BSD 2-Clause license. Note that NO WARRANTY is provided.
 * See "LICENSE_BSD2.txt" for details.
 *
 * @TAG(DATA61_BSD)
 */

#pragma once

/* An unmanaged binary semaphore; i.e. the caller stores the state related to
 * the semaphore itself. This can be useful in scenarios such as CAmkES, where
 * immediate concurrency means we have a race on initialising a managed
 * semaphore.
 */

#include <assert.h>
#include <sel4/sel4.h>
#include <stddef.h>
#include <platsupport/sync/atomic.h>

static inline int sync_bin_sem_bare_wait(seL4_CPtr notification, volatile int *value) {
    int oldval;
    int result = sync_atomic_decrement_safe(value, &oldval, __ATOMIC_ACQUIRE);
    if (result != 0) {
        /* Failed decrement; too many outstanding lock holders. */
        return -1;
    }
    if (oldval <= 0) {
        seL4_Wait(notification, NULL);
        /* Even though we performed an acquire barrier during the atomic
         * decrement we did not actually have the lock yet, so we have
         * to do another one now */
        __atomic_thread_fence(__ATOMIC_ACQUIRE);
    }
    return 0;
}

static inline int sync_bin_sem_bare_post(seL4_CPtr notification, volatile int *value) {
    /* We can do an "unsafe" increment here because we know we are the only
     * lock holder.
     */
    int val = sync_atomic_increment(value, __ATOMIC_RELEASE);
    assert(*value <= 1);
    if (val <= 0) {
        seL4_Signal(notification);
    }
    return 0;
}