mirror of
https://gitlab.com/qemu-project/qemu
synced 2024-09-06 18:05:37 +00:00
![Stefan Hajnoczi](/assets/img/avatar_default.png)
QemuMutex does not guarantee fairness and cannot be acquired recursively: Fairness means each locker gets a turn and the scheduler cannot cause starvation. Recursive locking is useful for composition, it allows a sequence of locking operations to be invoked atomically by acquiring the lock around them. This patch adds RFifoLock, a recursive lock that guarantees FIFO order. Its first user is added in the next patch. RFifoLock has one additional feature: it can be initialized with an optional contention callback. The callback is invoked whenever a thread must wait for the lock. For example, it can be used to poke the current owner so that they release the lock soon. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
79 lines
1.9 KiB
C
79 lines
1.9 KiB
C
/*
|
|
* 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 <assert.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);
|
|
}
|