Messages in this thread Patch in this message | | | Date | Wed, 30 Jun 2021 17:35:18 +0200 | From | Peter Zijlstra <> | Subject | [RFC][PATCH 2/4] locking/mutex: Fix HANDOFF condition |
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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@windriver.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- kernel/locking/mutex.c | 15 +++++---------- 1 file changed, 5 insertions(+), 10 deletions(-)
--- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -919,7 +919,6 @@ __mutex_lock_common(struct mutex *lock, struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx) { struct mutex_waiter waiter; - bool first = false; struct ww_mutex *ww; int ret; @@ -998,6 +997,8 @@ __mutex_lock_common(struct mutex *lock, 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 @@ -1026,15 +1027,9 @@ __mutex_lock_common(struct mutex *lock, 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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