From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Ding Tianhong <dingtianhong@huawei.com>,
Jason Low <jason.low2@hpe.com>,
Davidlohr Bueso <dave@stgolabs.net>,
"Paul E. McKenney" <paulmck@us.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <Will.Deacon@arm.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH v3 3/3] locking/mutex: Avoid missed wakeup of mutex waiter
Date: Tue, 29 Mar 2016 18:36:54 +0200 [thread overview]
Message-ID: <20160329163654.GM3408@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1458668804-10138-4-git-send-email-Waiman.Long@hpe.com>
On Tue, Mar 22, 2016 at 01:46:44PM -0400, Waiman Long wrote:
> The current mutex code sets count to -1 and then sets the task
> state. This is the same sequence that the mutex unlock path is checking
> count and task state. That could lead to a missed wakeup even though
> the problem will be cleared when a new waiter enters the waiting queue.
>
> This patch reverses the order in the locking slowpath so that the task
> state is set first before setting the count. This should eliminate
> the potential missed wakeup and improve latency.
Is it really a problem though?
So the 'race' is __mutex_lock_common() against
__mutex_fastpath_unlock(), and that is fully serialized as per the
atomic instructions. Either the fast unlock path does 1->0 and the lock
acquires, or the lock sets -1, at which the unlock fails and enters
__mutex_unlock_common_slowpath, which is fully serialised against
__mutex_lock_common by the lock->wait_lock.
I agree that the code is nicer after your patch, but I don't actually
see a problem.
next prev parent reply other threads:[~2016-03-29 16:37 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-22 17:46 [PATCH v3 0/3] locking/mutex: Enable optimistic spinning of lock waiter Waiman Long
2016-03-22 17:46 ` [PATCH v3 1/3] locking/mutex: Add waiter parameter to mutex_optimistic_spin() Waiman Long
2016-03-22 17:46 ` [PATCH v3 2/3] locking/mutex: Enable optimistic spinning of woken task in wait queue Waiman Long
2016-03-29 15:39 ` Peter Zijlstra
2016-03-29 16:42 ` Peter Zijlstra
2016-03-31 20:40 ` Waiman Long
2016-03-31 20:37 ` Waiman Long
2016-03-22 17:46 ` [PATCH v3 3/3] locking/mutex: Avoid missed wakeup of mutex waiter Waiman Long
2016-03-29 16:36 ` Peter Zijlstra [this message]
2016-03-31 20:39 ` Waiman Long
2016-07-18 17:59 ` [PATCH v3 0/3] locking/mutex: Enable optimistic spinning of lock waiter Jason Low
2016-07-18 20:50 ` Waiman Long
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