From: Mathieu Desnoyers Date: Tue, 17 Mar 2015 21:53:21 +0000 (-0400) Subject: Fix: compat_futex_noasync race condition X-Git-Tag: v0.9.0~64 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=db21eff94739900b1ab51d231dfb281ead0a0fa5;p=userspace-rcu.git Fix: compat_futex_noasync race condition The Userspace RCU compatibility layer around sys_futex has a race condition which makes pretty much all "benchmark" tests hang pretty quickly on non-Linux systems (tested on Mac OS X). I narrowed it down to a bug in compat_futex_noasync: this compat layer uses a single pthread mutex and condition variable for all callers, independently of their uaddr. The FUTEX_WAKE performs a pthread cond broadcast to all waiters. FUTEX_WAIT must then compare *uaddr with val to see which thread has been awakened. Unfortunately, the check was not done again after each return from pthread_cond_wait(), thus causing the race. This race affects threads using the futex_noasync() compatibility layer concurrently, thus it affects only on non-Linux systems. Signed-off-by: Mathieu Desnoyers --- diff --git a/compat_futex.c b/compat_futex.c index e7342fe..9a08624 100644 --- a/compat_futex.c +++ b/compat_futex.c @@ -72,17 +72,26 @@ int compat_futex_noasync(int32_t *uaddr, int op, int32_t val, assert(!ret); switch (op) { case FUTEX_WAIT: - if (*uaddr != val) - goto end; - pthread_cond_wait(&__urcu_compat_futex_cond, &__urcu_compat_futex_lock); + /* + * Wait until *uaddr is changed to something else than "val". + * Comparing *uaddr content against val figures out which + * thread has been awakened. + */ + while (*uaddr == val) + pthread_cond_wait(&__urcu_compat_futex_cond, + &__urcu_compat_futex_lock); break; case FUTEX_WAKE: + /* + * Each wake is sending a broadcast, thus attempting wakeup of + * all awaiting threads, independently of their respective + * uaddr. + */ pthread_cond_broadcast(&__urcu_compat_futex_cond); break; default: gret = -EINVAL; } -end: ret = pthread_mutex_unlock(&__urcu_compat_futex_lock); assert(!ret); return gret;