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2da61b671e
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
/*
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* Recursive FIFO lock
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*
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* Copyright Red Hat, Inc. 2013
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*
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* Authors:
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* Stefan Hajnoczi <stefanha@redhat.com>
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*
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* This work is licensed under the terms of the GNU LGPL, version 2 or later.
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* See the COPYING.LIB file in the top-level directory.
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*
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*/
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#include <assert.h>
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#include "qemu/rfifolock.h"
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void rfifolock_init(RFifoLock *r, void (*cb)(void *), void *opaque)
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{
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qemu_mutex_init(&r->lock);
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r->head = 0;
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r->tail = 0;
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qemu_cond_init(&r->cond);
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r->nesting = 0;
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r->cb = cb;
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r->cb_opaque = opaque;
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}
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void rfifolock_destroy(RFifoLock *r)
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{
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qemu_cond_destroy(&r->cond);
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qemu_mutex_destroy(&r->lock);
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}
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/*
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* Theory of operation:
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*
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* In order to ensure FIFO ordering, implement a ticketlock. Threads acquiring
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* the lock enqueue themselves by incrementing the tail index. When the lock
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* is unlocked, the head is incremented and waiting threads are notified.
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*
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* Recursive locking does not take a ticket since the head is only incremented
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* when the outermost recursive caller unlocks.
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*/
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void rfifolock_lock(RFifoLock *r)
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{
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qemu_mutex_lock(&r->lock);
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/* Take a ticket */
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unsigned int ticket = r->tail++;
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if (r->nesting > 0 && qemu_thread_is_self(&r->owner_thread)) {
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r->tail--; /* put ticket back, we're nesting */
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} else {
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while (ticket != r->head) {
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/* Invoke optional contention callback */
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if (r->cb) {
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r->cb(r->cb_opaque);
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}
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qemu_cond_wait(&r->cond, &r->lock);
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}
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}
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qemu_thread_get_self(&r->owner_thread);
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r->nesting++;
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qemu_mutex_unlock(&r->lock);
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}
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void rfifolock_unlock(RFifoLock *r)
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{
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qemu_mutex_lock(&r->lock);
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assert(r->nesting > 0);
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assert(qemu_thread_is_self(&r->owner_thread));
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if (--r->nesting == 0) {
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r->head++;
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qemu_cond_broadcast(&r->cond);
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}
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qemu_mutex_unlock(&r->lock);
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}
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