Re: Revert "aio: fix aio request leak when events are reaped by user space"

From: Dan Aloni
Date: Sun Aug 24 2014 - 14:48:32 EST

On Sun, Aug 24, 2014 at 02:05:31PM -0400, Benjamin LaHaise wrote:
> On Fri, Aug 22, 2014 at 09:51:10PM +0300, Dan Aloni wrote:
> > Ben, seems that the test program needs some twidling to make the bug
> > appear still by setting MAX_IOS to 256 (and it still passes on a
> > kernel with the original patch reverted). Under this condition the
> > ring buffer size remains 128 (here, 32*4 CPUs), and it is overrun on
> > the second attempt.
> Thanks for checking that. I've made some further changes to your test
> program to be able to support both userspace event reaping and using the
> io_getevents() syscall. I also added a --max-ios= parameter to allow
> using different values of MAX_IOS for testing. A copy of the modified
> source is available at .
> With this version of the patch, both means of reaping events now work
> corectly. The aio_bug.c code still needs to be added to libaio's set
> of tests, but that's a separate email.

I like your extension to the test program. I have a suggestion for
the integration inside libaio's harness, since the original issue
depended on the size of the ring buffer, let's have max_ios be picked
from {ring->nr + 1, ring->nr - 1, ring->nr}*{1,2,3,4} for the various
test cases. I guess Jeff who is maintaining it will have some ideas

> If you can have a quick look and acknowledge with your Signed-off-by, I'll
> add it to the commit and send a pull request. With these changes and the
> modified aio_bug.c, the test passes using various random values for
> max_ios, as well as both with and without --user_getevents validating that
> both regressions involved have now been fixed.

I haven't tested it yet due to lack of more time today (will give it a
try tomorrow), but I've reviewed and didn't find any problem.

Signed-off-by: Dan Aloni <dan@xxxxxxxxxxxx>

> ...
> > BTW, I am not an expert on this code so I am still not sure that
> > 'ctx->completed_events' wouldn't get wrong if for instance - one SMP
> > core does userspace event reaping and another calls io_submit(). Do
> > you think it would work alright in that case?
> This should be SMP safe: both ctx->completed_events and ctx->tail are
> protected ctx->completion_lock. Additionally, possible races with
> grabbing the value of ring->head should also be safe, and I added a
> comment explaining why. Does that address your concerns? Cheers,

Yes, thanks.

Dan Aloni
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