LXR qemu/util/rfifolock.c

   1/*
   2 * Recursive FIFO lock
   3 *
   4 * Copyright Red Hat, Inc. 2013
   5 *
   6 * Authors:
   7 *  Stefan Hajnoczi   <stefanha@redhat.com>
   8 *
   9 * This work is licensed under the terms of the GNU LGPL, version 2 or later.
  10 * See the COPYING.LIB file in the top-level directory.
  11 *
  12 */
  13
  14#include "qemu/osdep.h"
  15#include "qemu/rfifolock.h"
  16
  17void rfifolock_init(RFifoLock *r, void (*cb)(void *), void *opaque)
  18{
  19    qemu_mutex_init(&r->lock);
  20    r->head = 0;
  21    r->tail = 0;
  22    qemu_cond_init(&r->cond);
  23    r->nesting = 0;
  24    r->cb = cb;
  25    r->cb_opaque = opaque;
  26}
  27
  28void rfifolock_destroy(RFifoLock *r)
  29{
  30    qemu_cond_destroy(&r->cond);
  31    qemu_mutex_destroy(&r->lock);
  32}
  33
  34/*
  35 * Theory of operation:
  36 *
  37 * In order to ensure FIFO ordering, implement a ticketlock.  Threads acquiring
  38 * the lock enqueue themselves by incrementing the tail index.  When the lock
  39 * is unlocked, the head is incremented and waiting threads are notified.
  40 *
  41 * Recursive locking does not take a ticket since the head is only incremented
  42 * when the outermost recursive caller unlocks.
  43 */
  44void rfifolock_lock(RFifoLock *r)
  45{
  46    qemu_mutex_lock(&r->lock);
  47
  48    /* Take a ticket */
  49    unsigned int ticket = r->tail++;
  50
  51    if (r->nesting > 0 && qemu_thread_is_self(&r->owner_thread)) {
  52        r->tail--; /* put ticket back, we're nesting */
  53    } else {
  54        while (ticket != r->head) {
  55            /* Invoke optional contention callback */
  56            if (r->cb) {
  57                r->cb(r->cb_opaque);
  58            }
  59            qemu_cond_wait(&r->cond, &r->lock);
  60        }
  61        qemu_thread_get_self(&r->owner_thread);
  62    }
  63
  64    r->nesting++;
  65    qemu_mutex_unlock(&r->lock);
  66}
  67
  68void rfifolock_unlock(RFifoLock *r)
  69{
  70    qemu_mutex_lock(&r->lock);
  71    assert(r->nesting > 0);
  72    assert(qemu_thread_is_self(&r->owner_thread));
  73    if (--r->nesting == 0) {
  74        r->head++;
  75        qemu_cond_broadcast(&r->cond);
  76    }
  77    qemu_mutex_unlock(&r->lock);
  78}
  79