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

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Message-ID: <AANLkTikgsGHJ+q6=We_zPAivyABq+z2f6Atv6ZScLYOU@mail.gmail.com>
Date:	Tue, 18 Jan 2011 10:24:24 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	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 Tue, Jan 18, 2011 at 6:07 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Nick Piggin <npiggin@...il.com> writes:
>
>> On Sat, Jan 15, 2011 at 1:52 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
>>> Nick Piggin <npiggin@...il.com> writes:
>>>
>>>> Hi,
>>>>
>>>> While hunting down a bug in NFS's AIO, I believe I found this
>>>> buggy code...
>>>>
>>>> fs: aio fix rcu ioctx lookup
>>>>
>>>> aio-dio-invalidate-failure GPFs in aio_put_req from io_submit.
>>>>
>>>> lookup_ioctx doesn't implement the rcu lookup pattern properly.
>>>> rcu_read_lock does not prevent refcount going to zero, so we
>>>> might take a refcount on a zero count ioctx.
>>>
>>> So, does this patch fix the problem?  You didn't actually say....
>>
>> No, it seemd to be an NFS AIO problem, although it was a
>> slightly older kernel so I'll re test after -rc1 if I haven't heard
>> back about it.
>
> OK.
>
>> 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.
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