This is a note to let you know that I've just added the patch titled locking/mutex: Fix HANDOFF condition to the 5.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: locking-mutex-fix-handoff-condition.patch and it can be found in the queue-5.14 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 345bc2ed19e71b5a709c0f8096a51b3a7dce2816 Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Date: Wed Jun 30 17:35:18 2021 +0200 locking/mutex: Fix HANDOFF condition [ Upstream commit 048661a1f963e9517630f080687d48af79ed784c ] Yanfei reported that setting HANDOFF should not depend on recomputing @first, only on @first state. Which would then give: if (ww_ctx || !first) first = __mutex_waiter_is_first(lock, &waiter); if (first) __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); But because 'ww_ctx || !first' is basically 'always' and the test for first is relatively cheap, omit that first branch entirely. Reported-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Reviewed-by: Waiman Long <longman@xxxxxxxxxx> Reviewed-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20210630154114.896786297@xxxxxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index d2df5e68b503..fb30e1436dfb 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -928,7 +928,6 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx) { struct mutex_waiter waiter; - bool first = false; struct ww_mutex *ww; int ret; @@ -1007,6 +1006,8 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas set_current_state(state); for (;;) { + bool first; + /* * Once we hold wait_lock, we're serialized against * mutex_unlock() handing the lock off to us, do a trylock @@ -1035,15 +1036,9 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas spin_unlock(&lock->wait_lock); schedule_preempt_disabled(); - /* - * ww_mutex needs to always recheck its position since its waiter - * list is not FIFO ordered. - */ - if (ww_ctx || !first) { - first = __mutex_waiter_is_first(lock, &waiter); - if (first) - __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); - } + first = __mutex_waiter_is_first(lock, &waiter); + if (first) + __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); set_current_state(state); /*