linux-ext4 - Re: [patch] fix up lock order reversal in writeback

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Date:	Thu, 18 Nov 2010 17:00:00 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ted Ts'o <tytso@....edu>, Nick Piggin <npiggin@...nel.dk>,
	Eric Sandeen <sandeen@...hat.com>, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-btrfs@...r.kernel.org
Subject: Re: [patch] fix up lock order reversal in writeback

On Wed, Nov 17, 2010 at 07:29:00PM -0800, Andrew Morton wrote:
> On Wed, 17 Nov 2010 22:06:13 -0500 "Ted Ts'o" <tytso@....edu> wrote:
> 
> > On Wed, Nov 17, 2010 at 05:10:57PM +1100, Nick Piggin wrote:
> > > On Tue, Nov 16, 2010 at 11:05:52PM -0600, Eric Sandeen wrote:
> > > > On 11/16/10 10:38 PM, Nick Piggin wrote:
> > > > >> as for the locking problems ... sorry about that!
> > > > > 
> > > > > That's no problem. So is that an ack? :)
> > > > > 
> > > > 
> > > > I'd like to test it with the original case it was supposed to solve; will
> > > > do that tomorrow.
> > > 
> > > OK, but it shouldn't make much difference, unless there is a lot of
> > > strange activity happening on the sb (like mount / umount / remount /
> > > freeze / etc).
> > 
> > This makes sense to me as well.
> > 
> > Acked-by: "Theodore Ts'o" <tytso@....edu>
> > 
> > So how do we want to send this patch to Linus?  It's a writeback
> > change, so through some mm tree?
> 
> It's in my todo pile.  Even though the patch sucks, but not as much as
> its changelog does.  Am not particularly happy merging an alleged
> bugfix where the bug is, and I quote, "I saw a lock order warning on
> ext4 trigger".  I mean, wtf?  How is anyone supposed to review the code
> based on that??  Or to understand it a year from now?

Sorry bout the confusion, it was supposed to be "i_mutex", and then it
would have been a bit more obvious.


> When I get to it I'll troll this email thread and might be able to
> kludge together a description which might be able to fool people into
> thinking it makes sense.

"Lock order reversal between s_umount and i_mutex".

i_mutex nests inside s_umount in some writeback paths (it was the end
io handler to convert unwritten extents IIRC). But hmm, wouldn't that
be a bug? We aren't allowed to take i_mutex inside writeback, are we?

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