linux-kernel - Re: [PATCH v1 3/8] sched/completion: convert completions to use simple wait queues

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Message-ID: <561BAB2C.9020000@bmw-carit.de>
Date:	Mon, 12 Oct 2015 14:44:28 +0200
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 3/8] sched/completion: convert completions to use
 simple wait queues

On 10/12/2015 01:58 PM, Peter Zijlstra wrote:
> On Mon, Oct 12, 2015 at 12:03:06PM +0200, Daniel Wagner wrote:
>> On 10/12/2015 11:17 AM, Daniel Wagner wrote:
>>> On 09/09/2015 04:26 PM, Peter Zijlstra wrote:
>>>> On Wed, Sep 09, 2015 at 02:05:29PM +0200, Daniel Wagner wrote:
>>>>> @@ -50,10 +50,10 @@ void complete_all(struct completion *x)
>>>>>  {
>>>>>  	unsigned long flags;
>>>>>  
>>>>> -	spin_lock_irqsave(&x->wait.lock, flags);
>>>>> +	raw_spin_lock_irqsave(&x->wait.lock, flags);
>>>>>  	x->done += UINT_MAX/2;
>>>>> -	__wake_up_locked(&x->wait, TASK_NORMAL, 0);
>>>>> -	spin_unlock_irqrestore(&x->wait.lock, flags);
>>>>> +	swake_up_locked(&x->wait);
>>>>> +	raw_spin_unlock_irqrestore(&x->wait.lock, flags);
>>>>>  }
>>>>>  EXPORT_SYMBOL(complete_all);
>>>>
>>>> I don't think that's correct; __wake_up_locked(.nr=0) would wake all
>>>> waiters, where swake_up_locked() will only wake one.
>>>
>>> I read that x->done should be protected via wait.lock during the whole
>>> operation. swake_up_all() will release and reacquire the lock while
>>> processing the all waiters. So we need to get
>>>
>>> Could we play a trick like setting the highest bit in done for
>>> indicating the complete_all() operation. The UINT_MAX/2 update looks
>>> like do this by setting a value which has the biggest offset from 0 (but
>>> why adding instead of just going for assigning...).
>>
>>
>> I had something like this here in mind:
> 
> I'm not exactly sure what problem you're trying to solve here.. The fact
> that we cannot call swake_all() while holding &x->wait.lock, or the fact
> that complete_all() is typically called from a context which cannot do
> swake_all() either?

The first one.

> Note:
> 
> Documentation/scheduler/completion.txt:complete() and complete_all() can be called in hard-irq/atomic context safely.
> 
> Which is very much _NOT_ true of swake_all().

Heh and I thought I got this right.

Looks like completion.c cannot use swait here. Or do you have an idea
how to do it? I was thinking on deferring the wake all call from
hard-irq/atomic but I guess this something to avoided.

cheers,
daniel
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