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SubjectRe: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order
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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.
>

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