From: Peter Zijlstra <peterz@infradead.org>
To: mingo@redhat.com, longman@redhat.com, boqun.feng@gmail.com,
will@kernel.org
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
yanfei.xu@windriver.com
Subject: [RFC][PATCH 2/4] locking/mutex: Fix HANDOFF condition
Date: Wed, 30 Jun 2021 17:35:18 +0200 [thread overview]
Message-ID: <20210630154114.896786297@infradead.org> (raw)
In-Reply-To: 20210630153516.832731403@infradead.org
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);
/*
next prev parent reply other threads:[~2021-06-30 15:42 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-30 15:35 [RFC][PATCH 0/4] locking/mutex: Some HANDOFF fixes Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 1/4] locking/mutex: Use try_cmpxchg() Peter Zijlstra
2021-07-05 11:59 ` Xu, Yanfei
2021-07-05 14:00 ` Peter Zijlstra
2021-07-05 14:52 ` Xu, Yanfei
2021-07-05 15:07 ` [PATCH] Documentation/atomic_t: Document cmpxchg() vs try_cmpxchg() Peter Zijlstra
2021-07-05 15:21 ` Will Deacon
2021-07-05 15:25 ` Xu, Yanfei
2021-07-05 17:12 ` Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] locking/mutex: Use try_cmpxchg() tip-bot2 for Peter Zijlstra
2021-06-30 15:35 ` Peter Zijlstra [this message]
2021-07-08 8:42 ` [tip: locking/core] locking/mutex: Fix HANDOFF condition tip-bot2 for Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 3/4] locking/mutex: Introduce __mutex_trylock_or_handoff() Peter Zijlstra
2021-06-30 16:30 ` Waiman Long
2021-06-30 18:04 ` Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-06-30 15:35 ` [RFC][PATCH 4/4] locking/mutex: Add MUTEX_WARN_ON Peter Zijlstra
2021-07-08 8:42 ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2021-06-30 19:03 ` [RFC][PATCH 0/4] locking/mutex: Some HANDOFF fixes Waiman Long
2021-07-01 2:11 ` Xu, Yanfei
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