From 0d0c2f40344640a2a6942dda156509593f51db5d Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Tue, 15 Apr 2014 20:42:39 -0400 Subject: [PATCH] fix deadlock race in pthread_once at the end of successful pthread_once, there was a race window during which another thread calling pthread_once would momentarily change the state back from 2 (finished) to 1 (in-progress). in this case, the status was immediately changed back, but with no wake call, meaning that waiters which arrived during this short window could block forever. there are two possible fixes. one would be adding the wake to the code path where it was missing. but it's better just to avoid reverting the status at all, by using compare-and-swap instead of swap. --- src/thread/pthread_once.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/src/thread/pthread_once.c b/src/thread/pthread_once.c index 41872f16..e01f6d48 100644 --- a/src/thread/pthread_once.c +++ b/src/thread/pthread_once.c @@ -18,7 +18,7 @@ int pthread_once(pthread_once_t *control, void (*init)(void)) * 1 - another thread is running init; wait * 2 - another thread finished running init; just return */ - for (;;) switch (a_swap(control, 1)) { + for (;;) switch (a_cas(control, 0, 1)) { case 0: pthread_cleanup_push(undo, control); init(); @@ -31,7 +31,6 @@ int pthread_once(pthread_once_t *control, void (*init)(void)) __wait(control, &waiters, 1, 0); continue; case 2: - a_store(control, 2); return 0; } } -- 2.25.1