From 3e7e67c08d3e91752a1d49b8efc66989d234cf11 Mon Sep 17 00:00:00 2001 From: Konstantin Belousov Date: Tue, 8 Sep 2015 08:41:07 +0000 Subject: [PATCH] In the pthread_once(), if the initializer has already run, then the calling thread is supposed to see accesses issued by the initializer. This means that the read of the once_control->state variable should have an acquire semantic. Use atomic_thread_fence_acq() when the value read is ONCE_DONE, instead of straightforward atomic_load_acq(), to only put a barrier when needed (*). On the other hand, the updates of the once_control->state with the intermediate progress state do not need to synchronize with other state accesses, remove _acq suffix. Reviewed by: alc (previous version) Suggested by: alc (*) Sponsored by: The FreeBSD Foundation MFC after: 1 week --- lib/libthr/thread/thr_once.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/lib/libthr/thread/thr_once.c b/lib/libthr/thread/thr_once.c index 4f7037416c25..4a6b4d7a7cbb 100644 --- a/lib/libthr/thread/thr_once.c +++ b/lib/libthr/thread/thr_once.c @@ -68,13 +68,15 @@ _pthread_once(pthread_once_t *once_control, void (*init_routine) (void)) for (;;) { state = once_control->state; - if (state == ONCE_DONE) + if (state == ONCE_DONE) { + atomic_thread_fence_acq(); return (0); + } if (state == ONCE_NEVER_DONE) { - if (atomic_cmpset_acq_int(&once_control->state, state, ONCE_IN_PROGRESS)) + if (atomic_cmpset_int(&once_control->state, state, ONCE_IN_PROGRESS)) break; } else if (state == ONCE_IN_PROGRESS) { - if (atomic_cmpset_acq_int(&once_control->state, state, ONCE_WAIT)) + if (atomic_cmpset_int(&once_control->state, state, ONCE_WAIT)) _thr_umtx_wait_uint(&once_control->state, ONCE_WAIT, NULL, 0); } else if (state == ONCE_WAIT) { _thr_umtx_wait_uint(&once_control->state, state, NULL, 0);