[PATCH AUTOSEL 5.10 01/39] locking/mutex: Fix HANDOFF condition

Sasha Levin
2021-09-06T01:25:37Z

From: Peter Zijlstra <peterz@infradead.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@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>
---
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 15ac7c4bb111..86061901636c 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);
/*
--
2.30.2