Message-ID: <56FD8B1F.7070102@hpe.com>
Date: Thu, 31 Mar 2016 16:39:59 -0400
From: Waiman Long <waiman.long@hpe.com>
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To: Peter Zijlstra <peterz@infradead.org>
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
References: <1458668804-10138-1-git-send-email-Waiman.Long@hpe.com> <1458668804-10138-4-git-send-email-Waiman.Long@hpe.com> <20160329163654.GM3408@twins.programming.kicks-ass.net>
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On 03/29/2016 12:36 PM, Peter Zijlstra wrote:
> 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.

You are right again. I think I missed the spinlock serialization part 
from my analysis. So this patch isn't really necessary. I can withdraw 
it or mark it as a cleanup.

Cheers,
Longman