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

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Message-ID: <AANLkTin1+B_1CV-weALdS8EYJO60BJd0b7AGWzc0wrWr@mail.gmail.com>
Date:	Wed, 19 Jan 2011 09:17:23 +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 Wed, Jan 19, 2011 at 6:01 AM, Jan Kara <jack@...e.cz> wrote:
>  Hi,
>
> On Tue 18-01-11 10:24:24, Nick Piggin wrote:
>> On Tue, Jan 18, 2011 at 6:07 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> > Nick Piggin <npiggin@...il.com> writes:
>> >> Do you agree with the theoretical problem? I didn't try to
>> >> write a racer to break it yet. Inserting a delay before the
>> >> get_ioctx might do the trick.
>> >
>> > I'm not convinced, no.  The last reference to the kioctx is always the
>> > process, released in the exit_aio path, or via sys_io_destroy.  In both
>> > cases, we cancel all aios, then wait for them all to complete before
>> > dropping the final reference to the context.
>>
>> That wouldn't appear to prevent a concurrent thread from doing an
>> io operation that requires ioctx lookup, and taking the last reference
>> after the io_cancel thread drops the ref.
>>
>> > So, while I agree that what you wrote is better, I remain unconvinced of
>> > it solving a real-world problem.  Feel free to push it in as a cleanup,
>> > though.
>>
>> Well I think it has to be technically correct first. If there is indeed a
>> guaranteed ref somehow, it just needs a comment.
>  Hmm, the code in io_destroy() indeed looks fishy. We delete the ioctx
> from the hash table and set ioctx->dead which is supposed to stop
> lookup_ioctx() from finding it (see the !ctx->dead check in
> lookup_ioctx()). There's even a comment in io_destroy() saying:
>        /*
>         * Wake up any waiters.  The setting of ctx->dead must be seen
>         * by other CPUs at this point.  Right now, we rely on the
>         * locking done by the above calls to ensure this consistency.
>         */
> But since lookup_ioctx() is called without any lock or barrier nothing
> really seems to prevent the list traversal and ioctx->dead test to happen
> before io_destroy() and get_ioctx() after io_destroy().
>
> But wouldn't the right fix be to call synchronize_rcu() in io_destroy()?
> Because with your fix we could still return 'dead' ioctx and I don't think
> we are supposed to do that...

With my fix we won't oops, I was a bit concerned about ->dead,
yes but I don't know what semantics it is attempted to have there.

synchronize_rcu() in io_destroy()  does not prevent it from returning
as soon as lookup_ioctx drops the rcu_read_lock().

The dead=1 in io_destroy indeed doesn't guarantee a whole lot.
Anyone know?
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