From a5e42516a61ead33e5b98c6508547e1cbef0f8ac Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 30 Jun 2021 17:35:18 +0200 Subject: [PATCH] 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Waiman Long Reviewed-by: Yanfei Xu Link: https://lore.kernel.org/r/20210630154114.896786297@infradead.org Signed-off-by: Sasha Levin --- 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 c0c7784f074b..b02fff28221f 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -938,7 +938,6 @@ __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; @@ -1017,6 +1016,8 @@ __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 @@ -1045,15 +1046,9 @@ __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); /*