Fix: compat_futex_noasync race condition
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tue, 17 Mar 2015 21:53:21 +0000 (17:53 -0400)
committerMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tue, 17 Mar 2015 21:58:44 +0000 (17:58 -0400)
commitdb21eff94739900b1ab51d231dfb281ead0a0fa5
tree0bfb0fcadfcd6026b7b551638708bc91dc8fe5aa
parent8d11f7cd74968f5baffdbfde8b38d3197ef6865d
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 <mathieu.desnoyers@efficios.com>
compat_futex.c
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