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/*
* Recursive FIFO lock
*
* Copyright Red Hat, Inc. 2013
*
* Authors:
* Stefan Hajnoczi <stefanha@redhat.com>
*
* This work is licensed under the terms of the GNU LGPL, version 2 or later.
* See the COPYING.LIB file in the top-level directory.
*
*/
#include "qemu/osdep.h"
#include "qemu/rfifolock.h"
void rfifolock_init(RFifoLock *r, void (*cb)(void *), void *opaque)
{
qemu_mutex_init(&r->lock);
r->head = 0;
r->tail = 0;
qemu_cond_init(&r->cond);
r->nesting = 0;
r->cb = cb;
r->cb_opaque = opaque;
}
void rfifolock_destroy(RFifoLock *r)
{
qemu_cond_destroy(&r->cond);
qemu_mutex_destroy(&r->lock);
}
/*
* Theory of operation:
*
* In order to ensure FIFO ordering, implement a ticketlock. Threads acquiring
* the lock enqueue themselves by incrementing the tail index. When the lock
* is unlocked, the head is incremented and waiting threads are notified.
*
* Recursive locking does not take a ticket since the head is only incremented
* when the outermost recursive caller unlocks.
*/
void rfifolock_lock(RFifoLock *r)
{
qemu_mutex_lock(&r->lock);
/* Take a ticket */
unsigned int ticket = r->tail++;
if (r->nesting > 0 && qemu_thread_is_self(&r->owner_thread)) {
r->tail--; /* put ticket back, we're nesting */
} else {
while (ticket != r->head) {
/* Invoke optional contention callback */
if (r->cb) {
r->cb(r->cb_opaque);
}
qemu_cond_wait(&r->cond, &r->lock);
}
}
qemu_thread_get_self(&r->owner_thread);
r->nesting++;
qemu_mutex_unlock(&r->lock);
}
void rfifolock_unlock(RFifoLock *r)
{
qemu_mutex_lock(&r->lock);
assert(r->nesting > 0);
assert(qemu_thread_is_self(&r->owner_thread));
if (--r->nesting == 0) {
r->head++;
qemu_cond_broadcast(&r->cond);
}
qemu_mutex_unlock(&r->lock);
}
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