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

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Date:   Fri, 16 Dec 2016 19:12:50 +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 18:20, Peter Zijlstra wrote:
> On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai Hähnle wrote:
>>>> @@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>>>> 		spin_unlock_mutex(&lock->wait_lock, flags);
>>>> 		schedule_preempt_disabled();
>>>>
>>>> -		if (!first && __mutex_waiter_is_first(lock, &waiter)) {
>>>> +		if (use_ww_ctx && ww_ctx) {
>>>> +			/*
>>>> +			 * Always re-check whether we're in first position. We
>>>> +			 * don't want to spin if another task with a lower
>>>> +			 * stamp has taken our position.
>>>> +			 *
>>>> +			 * We also may have to set the handoff flag again, if
>>>> +			 * our position at the head was temporarily taken away.
>>>> +			 */
>>>> +			first = __mutex_waiter_is_first(lock, &waiter);
>>>> +
>>>> +			if (first)
>>>> +				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
>>>> +		} else if (!first && __mutex_waiter_is_first(lock, &waiter)) {
>>>> 			first = true;
>>>> 			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
>>>> 		}
>>>
>>> So the point is that !ww_ctx entries are 'skipped' during the insertion
>>> and therefore, if one becomes first, it must stay first?
>>
>> Yes. Actually, it should be possible to replace all the cases of use_ww_ctx
>> || first with ww_ctx. Similarly, all cases of use_ww_ctx && ww_ctx could be
>> replaced by just ww_ctx.
>
>
> I'm not seeing how "use_ww_ctx || first" -> "ww_ctx" works.

My bad, missing the '|| first'.


> And while
> "use_ww_ctx && ww_ctx" -> "ww_ctx" is correct, it didn't work right on
> some older GCCs, they choked on value propagation for ww_ctx and kept
> emitting code even if we passed in NULL. Hence use_ww_ctx.

Okay, I'll stick with use_ww_ctx. Thanks for the explanation.

Nicolai


> Arnd is now looking to raise the minimum supported GCC version, so maybe
> we should look at that again if he gets anywhere.
>