From: Mathieu Desnoyers Date: Thu, 23 Jun 2022 20:15:51 +0000 (-0400) Subject: Fix: futex wait: handle spurious futex wakeups X-Git-Url: https://git.liburcu.org/?a=commitdiff_plain;h=36962e16797b5ca590c54a385ca594e2859ef8c2;hp=36962e16797b5ca590c54a385ca594e2859ef8c2;p=lttng-tools.git Fix: futex wait: handle spurious futex wakeups Observed issue ============== The futex futex_nto1_wait() implements a futex wait/wakeup scheme identical to the liburcu workqueue code, which has an issue with spurious wakeups. A spurious wakeup on futex_nto1_wait can cause futex_nto1_wait to return with a futex state of -1, which is unexpected. futex_nto1_wait is used by the relayd live dispatcher thread, by the relayd main dispatcher thread, as well as by the sessiond dispatcher thread. Given that following a futex_nto1_wait returning due to a spurious wakeup futex_nto1_prepare will set the futex value to -1, things go back to normal for the following futex_nto1_wait calls. Therefore, the only impact of this issue is to spuriously use slightly more CPU time than strictly required. The effect is even shorter-lasting that in the liburcu counterparts because futex_nto1_prepare explicitly sets the futex state to -1 rather than use an atomic decrement, which immediately sets to state back to a consistent state. Cause ===== From futex(5): FUTEX_WAIT Returns 0 if the caller was woken up. Note that a wake-up can also be caused by common futex usage patterns in unrelated code that happened to have previously used the futex word's memory location (e.g., typical futex-based implementations of Pthreads mutexes can cause this under some conditions). Therefore, call‐ ers should always conservatively assume that a return value of 0 can mean a spurious wake-up, and use the futex word's value (i.e., the user-space synchronization scheme) to decide whether to continue to block or not. Solution ======== We therefore need to validate whether the value differs from -1 in user-space after the call to FUTEX_WAIT returns 0. Known drawbacks =============== None. Signed-off-by: Mathieu Desnoyers Signed-off-by: Jérémie Galarneau Change-Id: I8a1b6aaf77b6a2f941fd4f89b61bed71cf17906b ---