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locking/mutex: Fix HANDOFF condition

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[ 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@windriver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Yanfei Xu <yanfei.xu@windriver.com>
Link: https://lore.kernel.org/r/20210630154114.896786297@infradead.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Peter Zijlstra 2021-06-30 17:35:18 +02:00 committed by Greg Kroah-Hartman
parent cb83afdc0b
commit 97bc540bfb
1 changed files with 5 additions and 10 deletions
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15
kernel/locking/mutex.c
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@ -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 ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
{ {
struct mutex_waiter waiter; struct mutex_waiter waiter;
bool first = false;
struct ww_mutex *ww; struct ww_mutex *ww;
int ret; int ret;
@ -1017,6 +1016,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
set_current_state(state); set_current_state(state);
for (;;) { for (;;) {
bool first;
/* /*
* Once we hold wait_lock, we're serialized against * Once we hold wait_lock, we're serialized against
* mutex_unlock() handing the lock off to us, do a trylock * 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); spin_unlock(&lock->wait_lock);
schedule_preempt_disabled(); schedule_preempt_disabled();
/* first = __mutex_waiter_is_first(lock, &waiter);
* ww_mutex needs to always recheck its position since its waiter if (first)
* list is not FIFO ordered. __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
*/
if (ww_ctx || !first) {
first = __mutex_waiter_is_first(lock, &waiter);
if (first)
__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
}
set_current_state(state); set_current_state(state);
/* /*