linux-kernel - Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

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Message-ID: <98cfeb6e-f312-ba13-00b4-f5b125b24f8d@gmail.com>
Date:   Fri, 16 Dec 2016 19:11:41 +0100
From:   Nicolai Hähnle <nhaehnle@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Maarten Lankhorst <dev@...ankhorst.nl>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

On 16.12.2016 18:15, Peter Zijlstra wrote:
> On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai Hähnle wrote:
>> The concern about picking up a handoff that we didn't request is real,
>> though it cannot happen in the first iteration. Perhaps this __mutex_trylock
>> can be moved to the end of the loop? See below...
>
>
>>>> @@ -728,7 +800,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>>>> 		 * or we must see its unlock and acquire.
>>>> 		 */
>>>> 		if ((first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) ||
>>>> -		     __mutex_trylock(lock, first))
>>>> +		     __mutex_trylock(lock, use_ww_ctx || first))
>>>> 			break;
>>>>
>>>> 		spin_lock_mutex(&lock->wait_lock, flags);
>>
>> Change this code to:
>>
>> 		acquired = first &&
>> 		    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx,
>> 					  &waiter);
>> 		spin_lock_mutex(&lock->wait_lock, flags);
>> 		
>> 		if (acquired ||
>> 		    __mutex_trylock(lock, use_ww_ctx || first))
>> 			break;
>
> 			goto acquired;
>
> will work lots better.

Wasn't explicit enough, sorry. The idea was to get rid of the acquired 
label and change things so that all paths exit the loop with wait_lock 
held. That seems cleaner to me.


>> 	}
>>
>> This changes the trylock to always be under the wait_lock, but we previously
>> had that at the beginning of the loop anyway.
>
>> It also removes back-to-back
>> calls to __mutex_trylock when going through the loop;
>
> Yeah, I had that explicitly. It allows taking the mutex when
> mutex_unlock() is still holding the wait_lock.

mutex_optimistic_spin() already calls __mutex_trylock, and for the 
no-spin case, __mutex_unlock_slowpath() only calls wake_up_q() after 
releasing the wait_lock.

So I don't see the purpose of the back-to-back __mutex_trylocks, 
especially considering that if the first one succeeds, we immediately 
take the wait_lock anyway.

Nicolai



>> and for the first
>> iteration, there is a __mutex_trylock under wait_lock already before adding
>> ourselves to the wait list.
>
> Correct.
>