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

From: Chen, Kenneth W
Date: Tue Jan 02 2007 - 20:17:14 EST


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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