kern_sema.c

/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (C) 2001 Jason Evans <jasone@freebsd.org>.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice(s), this list of conditions and the following disclaimer as
 *    the first lines of this file unmodified other than the possible 
 *    addition of one or more copyright notices.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice(s), this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``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 COPYRIGHT HOLDER(S) 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.
 */

/*
 * Counting semaphores.
 *
 * Priority propagation will not generally raise the priority of semaphore
 * "owners" (a misnomer in the context of semaphores), so should not be relied
 * upon in combination with semaphores.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/ktr.h>
#include <sys/condvar.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/sema.h>

void
sema_init(struct sema *sema, int value, const char *description)
{

	KASSERT((value >= 0), ("%s(): negative value\n", __func__));

	bzero(sema, sizeof(*sema));
	mtx_init(&sema->sema_mtx, description, "sema backing lock",
	    MTX_DEF | MTX_NOWITNESS | MTX_QUIET);
	cv_init(&sema->sema_cv, description);
	sema->sema_value = value;

	CTR4(KTR_LOCK, "%s(%p, %d, \"%s\")", __func__, sema, value, description);
}

void
sema_destroy(struct sema *sema)
{

	CTR3(KTR_LOCK, "%s(%p) \"%s\"", __func__, sema,
	    cv_wmesg(&sema->sema_cv));

	KASSERT((sema->sema_waiters == 0), ("%s(): waiters\n", __func__));

	mtx_destroy(&sema->sema_mtx);
	cv_destroy(&sema->sema_cv);
}

void
_sema_post(struct sema *sema, const char *file, int line)
{

	mtx_lock(&sema->sema_mtx);
	sema->sema_value++;
	if (sema->sema_waiters && sema->sema_value > 0)
		cv_signal(&sema->sema_cv);

	CTR6(KTR_LOCK, "%s(%p) \"%s\" v = %d at %s:%d", __func__, sema,
	    cv_wmesg(&sema->sema_cv), sema->sema_value, file, line);

	mtx_unlock(&sema->sema_mtx);
}

void
_sema_wait(struct sema *sema, const char *file, int line)
{

	mtx_lock(&sema->sema_mtx);
	while (sema->sema_value == 0) {
		sema->sema_waiters++;
		cv_wait(&sema->sema_cv, &sema->sema_mtx);
		sema->sema_waiters--;
	}
	sema->sema_value--;

	CTR6(KTR_LOCK, "%s(%p) \"%s\" v = %d at %s:%d", __func__, sema,
	    cv_wmesg(&sema->sema_cv), sema->sema_value, file, line);

	mtx_unlock(&sema->sema_mtx);
}

int
_sema_timedwait(struct sema *sema, int timo, const char *file, int line)
{
	int error;

	mtx_lock(&sema->sema_mtx);

	/*
	 * A spurious wakeup will cause the timeout interval to start over.
	 * This isn't a big deal as long as spurious wakeups don't occur
	 * continuously, since the timeout period is merely a lower bound on how
	 * long to wait.
	 */
	for (error = 0; sema->sema_value == 0 && error == 0;) {
		sema->sema_waiters++;
		error = cv_timedwait(&sema->sema_cv, &sema->sema_mtx, timo);
		sema->sema_waiters--;
	}
	if (sema->sema_value > 0) {
		/* Success. */
		sema->sema_value--;
		error = 0;

		CTR6(KTR_LOCK, "%s(%p) \"%s\" v = %d at %s:%d", __func__, sema,
		    cv_wmesg(&sema->sema_cv), sema->sema_value, file, line);
	} else {
		CTR5(KTR_LOCK, "%s(%p) \"%s\" fail at %s:%d", __func__, sema,
		    cv_wmesg(&sema->sema_cv), file, line);
	}

	mtx_unlock(&sema->sema_mtx);
	return (error);
}

int
_sema_trywait(struct sema *sema, const char *file, int line)
{
	int ret;

	mtx_lock(&sema->sema_mtx);

	if (sema->sema_value > 0) {
		/* Success. */
		sema->sema_value--;
		ret = 1;

		CTR6(KTR_LOCK, "%s(%p) \"%s\" v = %d at %s:%d", __func__, sema,
		    cv_wmesg(&sema->sema_cv), sema->sema_value, file, line);
	} else {
		ret = 0;

		CTR5(KTR_LOCK, "%s(%p) \"%s\" fail at %s:%d", __func__, sema,
		    cv_wmesg(&sema->sema_cv), file, line);
	}

	mtx_unlock(&sema->sema_mtx);
	return (ret);
}

int
sema_value(struct sema *sema)
{
	int ret;

	mtx_lock(&sema->sema_mtx);
	ret = sema->sema_value;
	mtx_unlock(&sema->sema_mtx);
	return (ret);
}