src/pulsecore/once.c

   1 /***
   2   This file is part of PulseAudio.
   3
   4   Copyright 2006 Lennart Poettering
   5
   6   PulseAudio is free software; you can redistribute it and/or modify
   7   it under the terms of the GNU Lesser General Public License as published
   8   by the Free Software Foundation; either version 2.1 of the License,
   9   or (at your option) any later version.
  10
  11   PulseAudio is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with PulseAudio; if not, write to the Free Software
  18   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
  19   USA.
  20 ***/
  21
  22 #ifdef HAVE_CONFIG_H
  23 #include <config.h>
  24 #endif
  25
  26 #include <pulsecore/macro.h>
  27 #include <pulsecore/mutex.h>
  28
  29 #include "once.h"
  30
  31 pa_bool_t pa_once_begin(pa_once *control) {
  32     pa_mutex *m;
  33
  34     pa_assert(control);
  35
  36     if (pa_atomic_load(&control->done))
  37         return FALSE;
  38
  39     pa_atomic_inc(&control->ref);
  40
  41     /* Caveat: We have to make sure that the once func has completed
  42      * before returning, even if the once func is not actually
  43      * executed by us. Hence the awkward locking. */
  44
  45     for (;;) {
  46
  47         if ((m = pa_atomic_ptr_load(&control->mutex))) {
  48
  49             /* The mutex is stored in locked state, hence let's just
  50              * wait until it is unlocked */
  51             pa_mutex_lock(m);
  52
  53             pa_assert(pa_atomic_load(&control->done));
  54
  55             pa_once_end(control);
  56             return FALSE;
  57         }
  58
  59         pa_assert_se(m = pa_mutex_new(FALSE, FALSE));
  60         pa_mutex_lock(m);
  61
  62         if (pa_atomic_ptr_cmpxchg(&control->mutex, NULL, m))
  63             return TRUE;
  64
  65         pa_mutex_unlock(m);
  66         pa_mutex_free(m);
  67     }
  68 }
  69
  70 void pa_once_end(pa_once *control) {
  71     pa_mutex *m;
  72
  73     pa_assert(control);
  74
  75     pa_atomic_store(&control->done, 1);
  76
  77     pa_assert_se(m = pa_atomic_ptr_load(&control->mutex));
  78     pa_mutex_unlock(m);
  79
  80     if (pa_atomic_dec(&control->ref) <= 1) {
  81         pa_assert_se(pa_atomic_ptr_cmpxchg(&control->mutex, m, NULL));
  82         pa_mutex_free(m);
  83     }
  84 }
  85
  86 /* Not reentrant -- how could it be? */
  87 void pa_run_once(pa_once *control, pa_once_func_t func) {
  88     pa_assert(control);
  89     pa_assert(func);
  90
  91     if (pa_once_begin(control)) {
  92         func();
  93         pa_once_end(control);
  94     }
  95 }