diff options
authorRich Felker <>2014-10-10 18:20:33 -0400
committerRich Felker <>2015-03-30 01:34:13 -0400
commite3fa4300bf17d2b051bf21f451656af841404c2b (patch)
parent02ccece69864c006bafd9c318402195c9d48aa4d (diff)
fix missing barrier in pthread_once/call_once shortcut path
these functions need to be fast when the init routine has already run, since they may be called very often from code which depends on global initialization having taken place. as such, a fast path bypassing atomic cas on the once control object was used to avoid heavy memory contention. however, on archs with weakly ordered memory, the fast path failed to ensure that the caller actually observes the side effects of the init routine. preliminary performance testing showed that simply removing the fast path was not practical; a performance drop of roughly 85x was observed with 20 threads hammering the same once control on a 24-core machine. so the new explicit barrier operation from atomic.h is used to retain the fast path while ensuring memory visibility. performance may be reduced on some archs where the barrier actually makes a difference, but the previous behavior was unsafe and incorrect on these archs. future improvements to the implementation of a_barrier should reduce the impact. (cherry picked from commit df37d3960abec482e17fad2274a99b790f6cc08b) (edited not to depend on a_barrier, which is not available in 1.0.x)
1 files changed, 7 insertions, 2 deletions
diff --git a/src/thread/pthread_once.c b/src/thread/pthread_once.c
index e01f6d48..7b9ccf25 100644
--- a/src/thread/pthread_once.c
+++ b/src/thread/pthread_once.c
@@ -10,8 +10,13 @@ int pthread_once(pthread_once_t *control, void (*init)(void))
static int waiters;
- /* Return immediately if init finished before */
- if (*control == 2) return 0;
+ /* Return immediately if init finished before, but ensure that
+ * effects of the init routine are visible to the caller. */
+ if (*control == 2) {
+ /* Otherwise-useless cas just to get a barrier. */
+ a_cas(&(int){0},0,0);
+ return 0;
+ }
/* Try to enter initializing state. Three possibilities:
* 0 - we're the first or the other cancelled; run init