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

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Message-ID: <561B7A9A.3020904@bmw-carit.de>
Date:	Mon, 12 Oct 2015 11:17:14 +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 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...).

cheers,
daniel

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