src/pulsecore/aupdate.c

   1 /***
   2   This file is part of PulseAudio.
   3
   4   Copyright 2009 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
   8   published by the Free Software Foundation; either version 2.1 of the
   9   License, 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
  17   License 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 <pulse/xmalloc.h>
  27
  28 #include <pulsecore/semaphore.h>
  29 #include <pulsecore/macro.h>
  30 #include <pulsecore/mutex.h>
  31
  32 #include "aupdate.h"
  33
  34 #define MSB (1U << (sizeof(unsigned)*8U-1))
  35 #define WHICH(n) (!!((n) & MSB))
  36 #define COUNTER(n) ((n) & ~MSB)
  37
  38 struct pa_aupdate {
  39     pa_atomic_t read_lock;
  40     pa_mutex *write_lock;
  41     pa_semaphore *semaphore;
  42     pa_bool_t swapped;
  43 };
  44
  45 pa_aupdate *pa_aupdate_new(void) {
  46     pa_aupdate *a;
  47
  48     a = pa_xnew(pa_aupdate, 1);
  49     pa_atomic_store(&a->read_lock, 0);
  50     a->write_lock = pa_mutex_new(FALSE, FALSE);
  51     a->semaphore = pa_semaphore_new(0);
  52
  53     return a;
  54 }
  55
  56 void pa_aupdate_free(pa_aupdate *a) {
  57     pa_assert(a);
  58
  59     pa_mutex_free(a->write_lock);
  60     pa_semaphore_free(a->semaphore);
  61
  62     pa_xfree(a);
  63 }
  64
  65 unsigned pa_aupdate_read_begin(pa_aupdate *a) {
  66     unsigned n;
  67
  68     pa_assert(a);
  69
  70     /* Increase the lock counter */
  71     n = (unsigned) pa_atomic_inc(&a->read_lock);
  72
  73     /* When n is 0 we have about 2^31 threads running that all try to
  74      * access the data at the same time, oh my! */
  75     pa_assert(COUNTER(n)+1 > 0);
  76
  77     /* The uppermost bit tells us which data to look at */
  78     return WHICH(n);
  79 }
  80
  81 void pa_aupdate_read_end(pa_aupdate *a) {
  82     unsigned n;
  83
  84     pa_assert(a);
  85
  86     /* Decrease the lock counter */
  87     n = (unsigned) pa_atomic_dec(&a->read_lock);
  88
  89     /* Make sure the counter was valid */
  90     pa_assert(COUNTER(n) > 0);
  91
  92     /* Post the semaphore */
  93     pa_semaphore_post(a->semaphore);
  94 }
  95
  96 unsigned pa_aupdate_write_begin(pa_aupdate *a) {
  97     unsigned n;
  98
  99     pa_assert(a);
 100
 101     pa_mutex_lock(a->write_lock);
 102
 103     n = (unsigned) pa_atomic_load(&a->read_lock);
 104
 105     a->swapped = FALSE;
 106
 107     return !WHICH(n);
 108 }
 109
 110 unsigned pa_aupdate_write_swap(pa_aupdate *a) {
 111     unsigned n;
 112
 113     pa_assert(a);
 114
 115     for (;;) {
 116         n = (unsigned) pa_atomic_load(&a->read_lock);
 117
 118         /* If the read counter is > 0 wait; if it is 0 try to swap the lists */
 119         if (COUNTER(n) > 0)
 120             pa_semaphore_wait(a->semaphore);
 121         else if (pa_atomic_cmpxchg(&a->read_lock, (int) n, (int) (n ^ MSB)))
 122             break;
 123     }
 124
 125     a->swapped = TRUE;
 126
 127     return WHICH(n);
 128 }
 129
 130 void pa_aupdate_write_end(pa_aupdate *a) {
 131     pa_assert(a);
 132
 133     if (!a->swapped)
 134         pa_aupdate_write_swap(a);
 135
 136     pa_mutex_unlock(a->write_lock);
 137 }