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once: Fix race causing pa_once to sometimes run twice

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There was a race in the existing code that could cause the pa_once code
to be run twice, see:
http://lists.freedesktop.org/archives/pulseaudio-discuss/2012-April/013354.html

Therefore the existing implementation was rewritten to instead look like
the reference implementation here:
http://www.hpl.hp.com/research/linux/atomic_ops/example.php4

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
This commit is contained in:
David Henningsson 2012-05-18 22:29:41 +02:00
parent 4c65e58325
commit 1ba655560d
2 changed files with 17 additions and 34 deletions
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37
src/pulsecore/once.c
View file

@ -24,46 +24,33 @@
#endif
#include <pulsecore/macro.h>
#include <pulsecore/mutex.h>
#include "once.h"
/* See http://www.hpl.hp.com/research/linux/atomic_ops/example.php4 for the
* reference algorithm used here. */
pa_bool_t pa_once_begin(pa_once *control) {
pa_mutex *m;
pa_assert(control);
if (pa_atomic_load(&control->done))
return FALSE;
pa_atomic_inc(&control->ref);
/* Caveat: We have to make sure that the once func has completed
* before returning, even if the once func is not actually
* executed by us. Hence the awkward locking. */
for (;;) {
pa_mutex *m;
if ((m = pa_atomic_ptr_load(&control->mutex))) {
/* The mutex is stored in locked state, hence let's just
* wait until it is unlocked */
m = pa_static_mutex_get(&control->mutex, FALSE, FALSE);
pa_mutex_lock(m);
pa_assert(pa_atomic_load(&control->done));
pa_once_end(control);
if (pa_atomic_load(&control->done)) {
pa_mutex_unlock(m);
return FALSE;
}
pa_assert_se(m = pa_mutex_new(FALSE, FALSE));
pa_mutex_lock(m);
if (pa_atomic_ptr_cmpxchg(&control->mutex, NULL, m))
return TRUE;
pa_mutex_unlock(m);
pa_mutex_free(m);
}
}
void pa_once_end(pa_once *control) {
@ -71,15 +58,11 @@ void pa_once_end(pa_once *control) {
pa_assert(control);
pa_assert(!pa_atomic_load(&control->done));
pa_atomic_store(&control->done, 1);
pa_assert_se(m = pa_atomic_ptr_load(&control->mutex));
m = pa_static_mutex_get(&control->mutex, FALSE, FALSE);
pa_mutex_unlock(m);
if (pa_atomic_dec(&control->ref) <= 1) {
pa_assert_se(pa_atomic_ptr_cmpxchg(&control->mutex, m, NULL));
pa_mutex_free(m);
}
}
/* Not reentrant -- how could it be? */

8
src/pulsecore/once.h
View file

@ -23,16 +23,16 @@
***/
#include <pulsecore/atomic.h>
#include <pulsecore/mutex.h>
typedef struct pa_once {
pa_atomic_ptr_t mutex;
pa_atomic_t ref, done;
pa_static_mutex mutex;
pa_atomic_t done;
} pa_once;
#define PA_ONCE_INIT \
{ \
.mutex = PA_ATOMIC_PTR_INIT(NULL), \
.ref = PA_ATOMIC_INIT(0), \
.mutex = PA_STATIC_MUTEX_INIT, \
.done = PA_ATOMIC_INIT(0) \
}