linux-kernel - Re: [patch] fs: aio fix rcu lookup

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Date:	Thu, 20 Jan 2011 04:37:55 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] fs: aio fix rcu lookup

On Thu, Jan 20, 2011 at 3:50 AM, Jan Kara <jack@...e.cz> wrote:
> On Thu 20-01-11 03:03:23, Nick Piggin wrote:
>> On Thu, Jan 20, 2011 at 12:21 AM, Jan Kara <jack@...e.cz> wrote:
>> >  Well, we are not required to cancel all the outstanding AIO because of the
>> > API requirement, that's granted. But we must do it because of the way how
>> > the code is written. Outstanding IO requests reference ioctx but they are
>> > not counted in ctx->users but in ctx->reqs_active. So the code relies on
>> > the fact that the reference held by the hash table protects ctx from being
>> > freed and io_destroy() waits for requests before dropping the last
>> > reference to ctx. But there's the second race I describe making it possible
>> > for new IO to be created after io_destroy() has waited for all IO to
>> > finish...
>>
>> Yes there is that race too I agree. I just didn't follow through the code far
>> enough to see it was a problem -- I thought it was by design.
>>
>> I'd like to solve it without synchronize_rcu() though.
>  Ah, OK. I don't find io_destroy() performance critical but I can

Probably not performance critical, but it could be a very
large slowdown so somebody might complain.

> understand that you need not like synchronize_rcu() there. ;) Then it
> should be possible to make IO requests count in ctx->users which would
> solve the race as well. We'd just have to be prepared that request
> completion might put the last reference to ioctx and free it but that
> shouldn't be an issue. Do you like that solution better?

I think so, if it can be done without slowing things down
and adding locks or atomics if possible.
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