[PATCH 4.19 032/293] locking/mutex: Fix HANDOFF condition
From: Greg Kroah-Hartman
Date: Mon Sep 20 2021 - 13:38:53 EST
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
[ 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@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
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
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 354151fef06a..fbc62d360419 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -911,7 +911,6 @@ __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;
@@ -986,6 +985,8 @@ __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
@@ -1014,15 +1015,9 @@ __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