linux-kernel - Re: next-20130117 - kernel BUG with aio

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Date:	Tue, 29 Jan 2013 14:41:39 +0100
From:	Jan Kara <jack@...e.cz>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, jack@...e.cz,
	tytso@....edu, Valdis.Kletnieks@...edu,
	Hillf Danton <dhillf@...il.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org, zab@...bo.net,
	linux-fsdevel@...r.kernel.org
Subject: Re: next-20130117 - kernel BUG with aio

On Thu 24-01-13 14:13:52, Kent Overstreet wrote:
> On Thu, Jan 24, 2013 at 01:27:59PM -0800, Andrew Morton wrote:
> > Please also take a look at Jan's recent
> > http://www.spinics.net/lists/linux-fsdevel/msg61738.html and have a
> > think about how this plays with your patchset.
> 
> I can't think of any possible interactions - none of my aio stuff messes
> with the way the fput() happens; the aio code does call fput() when the
> kiocb is freed and my patches do touch _that_ code but none of the
> behaviour there changes.
> 
> Might be worth documenting this though, I can't think of any reason it'd
> be obvious looking at the aio code that the fput() has to happen after
> aio_complete(). As with the bugs I just sent you patches for it's not
> terribly clear who owns what in the kiocb when.
> 
> Reading those patches though - the main change is to call
> inode_dio_done() before calling aio_complete(). All inode_dio_done()
> does though is issue a wakeup - to whatever called inode_dio_wait().
  inode_dio_done() does a decrement and wakeup.

> That means whatever called inode_dio_wait() needs its own ref on the
> inode, and from a cursory glance at the code it is _not_ at all clear to
> me that's the case - if inode_dio_wait() is merely finishing things for
> that specific IO that need to be done in process context, I can easily
> imagine it not being the case.
> 
> Assuming whatever does call inode_dio_wait() does have its own ref,
> there was only a real use after free when nothing was waiting on the
> inode.
  Well, but there doesn't have to be any waiter... If there is, it had
better have it's own ref, that's for sure.

> Similarly for the ext4 code to write unwritten extents - and having seen
> and helped chase a bug in that code before, that code _definitely_ needs
> auditing.
  Agreed. That code is a mess. I'm cleaning up some of it but it's not
easy.

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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