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Date: Sun, 5 Sep 2021 21:19:05 -0400 From: Sasha Levin <sashal@...nel.org> To: linux-kernel@...r.kernel.org, stable@...r.kernel.org Cc: Peter Zijlstra <peterz@...radead.org>, Yanfei Xu <yanfei.xu@...driver.com>, Waiman Long <longman@...hat.com>, Sasha Levin <sashal@...nel.org> Subject: [PATCH AUTOSEL 5.14 01/47] locking/mutex: Fix HANDOFF condition From: Peter Zijlstra <peterz@...radead.org> [ 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@...driver.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org> Reviewed-by: Waiman Long <longman@...hat.com> Reviewed-by: Yanfei Xu <yanfei.xu@...driver.com> Link: https://lore.kernel.org/r/20210630154114.896786297@infradead.org Signed-off-by: Sasha Levin <sashal@...nel.org> --- kernel/locking/mutex.c | 15 +++++---------- 1 file changed, 5 insertions(+), 10 deletions(-) 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); /* -- 2.30.2
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