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 <assert.h>
  15 #include "qemu/rfifolock.h"
  16
  17 void 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
  28 void 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  */
  44 void 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     }
  62
  63     qemu_thread_get_self(&r->owner_thread);
  64     r->nesting++;
  65     qemu_mutex_unlock(&r->lock);
  66 }
  67
  68 void 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 }