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

[Daniel Wagner <daniel.wagner@bmw-carit.de>]

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