The following commit has been merged into the locking/core branch of tip: Commit-ID: 048661a1f963e9517630f080687d48af79ed784c Gitweb: https://git.kernel.org/tip/048661a1f963e9517630f080687d48af79ed784c Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> AuthorDate: Wed, 30 Jun 2021 17:35:18 +02:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Wed, 07 Jul 2021 13:53:24 +02:00 locking/mutex: Fix HANDOFF condition 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 --- 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 cab7163..8c3d499 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -909,7 +909,6 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx) { struct mutex_waiter waiter; - bool first = false; struct ww_mutex *ww; int ret; @@ -988,6 +987,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 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 @@ -1016,15 +1017,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 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); /*
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