linux-kernel - RE: [patch] aio: add per task aio wait event condition

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Date:	Tue, 2 Jan 2007 17:16:55 -0800
From:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
To:	"'Zach Brown'" <zach.brown@...cle.com>
Cc:	"'Andrew Morton'" <akpm@...l.org>, <linux-aio@...ck.org>,
	<linux-kernel@...r.kernel.org>,
	"'Benjamin LaHaise'" <bcrl@...ck.org>, <suparna@...ibm.com>
Subject: RE: [patch] aio: add per task aio wait event condition

Zach Brown wrote on Tuesday, January 02, 2007 4:49 PM
> On Dec 29, 2006, at 6:31 PM, Chen, Kenneth W wrote:
> > This patch adds a wait condition to the wait queue and only wake-up
> > process when that condition meets.  And this condition is added on a
> > per task base for handling multi-threaded app that shares single  
> > ioctx.
> 
> But only one of the waiting tasks is tested, the one at the head of  
> the list.  It looks like this change could starve a io_getevents()  
> with a low min_nr in the presence of another io_getevents() with a  
> larger min_nr.
> 
> > +	if (waitqueue_active(&ctx->wait)) {
> > +		struct aio_wait_queue *wait;
> > +		wait = container_of(ctx->wait.task_list.next,
> > +				    struct aio_wait_queue, wait.task_list);
> > +		if (nr_evt >= wait->nr_wait)
> > +			wake_up(&ctx->wait);
> > +	}
> 
> First is the fear of starvation as mentioned previously.
> 
> issue 2 ops
> first io_getevents sleeps with a min_nr of 2
> second io_getevents sleeps with min_nr of 3
> 2 ops complete but only test the second sleeper's min_nr of 3
> first sleeper twiddles thumbs

That is not possible because when multiple tasks waiting for events, they
enter the wait queue in FIFO order, prepare_to_wait_exclusive() does
__add_wait_queue_tail().  So first io_getevents() with min_nr of 2 will
be woken up when 2 ops completes.

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