linux-kernel - Re: [PATCH] locking/mutex: Avoid spinner vs waiter starvation

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Message-ID: <56B27A37.6070105@hpe.com>
Date:	Wed, 03 Feb 2016 17:07:51 -0500
From:	Waiman Long <waiman.long@....com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH] locking/mutex: Avoid spinner vs waiter starvation

On 02/02/2016 04:19 PM, Davidlohr Bueso wrote:
> On Mon, 01 Feb 2016, Peter Zijlstra wrote:
>
>> Subject: locking/mutex: Avoid spinner vs waiter starvation
>> From: Peter Zijlstra <peterz@...radead.org>
>> Date: Fri, 22 Jan 2016 12:06:53 +0100
>>
>> Ding Tianhong reported that under his load the optimistic spinners
>> would totally starve a task that ended up on the wait list.
>>
>> Fix this by ensuring the top waiter also partakes in the optimistic
>> spin queue.
>>
>> There are a few subtle differences between the assumed state of
>> regular optimistic spinners and those already on the wait list, which
>> result in the @acquired complication of the acquire path.
>>
>> Most notable are:
>>
>> - waiters are on the wait list and need to be taken off
>> - mutex_optimistic_spin() sets the lock->count to 0 on acquire
>>   even though there might be more tasks on the wait list.
>
> Right, the main impact I see with these complications are that the
> window of when a waiter takes the lock via spinning and then acquires
> the wait_lock to remove itself from the list, will allow an unlock
> thread to set the lock as available in the fastpath which could in
> turn allow a third thread the steal the lock. With high contention,
> this window will be come obviously larger as we contend for the
> wait_lock.
>
> CPU-0                                  CPU-1            CPU-3
> __mutex_lock_common              mutex_optimistic_spin
>   (->count now 0)
>             __mutex_fastpath_unlock
>             (->count now 1)                 __mutex_fastpath_lock
>                                       (stolen)
>
> spin_lock_mutex(&lock->wait_lock, flags);
>
> But we've always been bad when it comes to counter and waiters.
>
> Thanks,
> Davidlohr

I don't quite get how that can happen. CPU0 cannot change the count to 0 
unless CPU1, the lock holder, does the unlock first. Once CPU0 sees a 
count of 1 and change it to 0, it is the lock holder and there can be no 
other CPU that can do the unlock.

Cheers,
Longman