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

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Message-ID: <x49tyh7mjr6.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 17 Jan 2011 14:07:41 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Nick Piggin <npiggin@...il.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

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.

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.

Cheers,
Jeff
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