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/*
* 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.
*
* $FreeBSD$
*/
/*
* Shared/exclusive locks. This implementation assures deterministic lock
* granting behavior, so that slocks and xlocks are interleaved.
*
* Priority propagation will not generally raise the priority of lock holders,
* so should not be relied upon in combination with sx locks.
*
* The witness code can not detect lock cycles.
*
* slock --> xlock (deadlock)
* slock --> slock (slock recursion, not fatal)
* xlock --> xlock (deadlock)
* xlock --> slock (deadlock)
*/
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/ktr.h>
#include <sys/condvar.h>
#include <sys/mutex.h>
#include <sys/sx.h>
void
sx_init(struct sx *sx, const char *description)
{
mtx_init(&sx->sx_lock, description, MTX_DEF);
sx->sx_cnt = 0;
cv_init(&sx->sx_shrd_cv, description);
sx->sx_shrd_wcnt = 0;
cv_init(&sx->sx_excl_cv, description);
sx->sx_excl_wcnt = 0;
}
void
sx_destroy(struct sx *sx)
{
KASSERT((sx->sx_cnt == 0 && sx->sx_shrd_wcnt == 0 && sx->sx_excl_wcnt ==
0), ("%s: holders or waiters\n", __FUNCTION__));
mtx_destroy(&sx->sx_lock);
cv_destroy(&sx->sx_shrd_cv);
cv_destroy(&sx->sx_excl_cv);
}
void
sx_slock(struct sx *sx)
{
mtx_lock(&sx->sx_lock);
/*
* Loop in case we lose the race for lock acquisition.
*/
while (sx->sx_cnt < 0) {
sx->sx_shrd_wcnt++;
cv_wait(&sx->sx_shrd_cv, &sx->sx_lock);
sx->sx_shrd_wcnt--;
}
/* Acquire a shared lock. */
sx->sx_cnt++;
mtx_unlock(&sx->sx_lock);
}
void
sx_xlock(struct sx *sx)
{
mtx_lock(&sx->sx_lock);
/* Loop in case we lose the race for lock acquisition. */
while (sx->sx_cnt != 0) {
sx->sx_excl_wcnt++;
cv_wait(&sx->sx_excl_cv, &sx->sx_lock);
sx->sx_excl_wcnt--;
}
/* Acquire an exclusive lock. */
sx->sx_cnt--;
mtx_unlock(&sx->sx_lock);
}
void
sx_sunlock(struct sx *sx)
{
mtx_lock(&sx->sx_lock);
KASSERT((sx->sx_cnt > 0), ("%s: lacking slock\n", __FUNCTION__));
/* Release. */
sx->sx_cnt--;
/*
* If we just released the last shared lock, wake any waiters up, giving
* exclusive lockers precedence. In order to make sure that exclusive
* lockers won't be blocked forever, don't wake shared lock waiters if
* there are exclusive lock waiters.
*/
if (sx->sx_excl_wcnt > 0) {
if (sx->sx_cnt == 0)
cv_signal(&sx->sx_excl_cv);
} else if (sx->sx_shrd_wcnt > 0)
cv_broadcast(&sx->sx_shrd_cv);
mtx_unlock(&sx->sx_lock);
}
void
sx_xunlock(struct sx *sx)
{
mtx_lock(&sx->sx_lock);
KASSERT((sx->sx_cnt == -1), ("%s: lacking xlock\n", __FUNCTION__));
/* Release. */
sx->sx_cnt++;
/*
* Wake up waiters if there are any. Give precedence to slock waiters.
*/
if (sx->sx_shrd_wcnt > 0)
cv_broadcast(&sx->sx_shrd_cv);
else if (sx->sx_excl_wcnt > 0)
cv_signal(&sx->sx_excl_cv);
mtx_unlock(&sx->sx_lock);
}
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