linux-kernel - Re: ext3fs: umount+sync not enough to guarantee metadata-on-disk

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Date:	Mon, 11 Jun 2007 13:14:57 +0200
From:	Jan Kara <jack@...e.cz>
To:	Mark Lord <lkml@....ca>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Chuck Ebbert <cebbert@...hat.com>,
	Stephen Tweedie <sct@...hat.com>,
	Theodore Ts'o <tytso@....edu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: ext3fs: umount+sync not enough to guarantee metadata-on-disk

> Andrew Morton wrote:
> >On Thu, 07 Jun 2007 12:11:58 -0400
> >Chuck Ebbert <cebbert@...hat.com> wrote:
> >
> >>On 06/07/2007 11:41 AM, Andrew Morton wrote:
> >>>>   mount /var/lib/mythtv -oremount,ro
> >>>>   sync
> >>>>   umount /var/lib/mythtv
> >>>Did this succeed?  If the application is still truncating that file, the
> >>>umount should have failed.
> >>Shouldn't sync should wait for truncate to finish?
> >
> >I can't think of anything in there at present which would cause that to
> >happen, and it's not immediately obvious how we _could_ make it happen - we
> >have an inode which potentially has no dirty pages and which is itself
> >clean.  The truncate can span multiple journal commits, so forcing a
> >journal commit in sync() won't necessarily block behind the truncate.
> >
> >I guess we could ask sync to speculatively take and release every inode's
> >i_mutex or something.  But even that would involve quite some hoop-jumping
> >due to those infuriating spinlock-protected list_heads on the superblock.
> >
> >hmm.
> 
> Okay, I added more instrumentation and retested today.
> 
> Good and Bad.
> The umount does indeed fail while the massive unlink is happening,
> so I can just loop on that a few times before giving up.
> 
> But.. the earlier "remount,ro".. well.. I don't know what it does.
> I did get it to lock up solid, though.. hung on the "remount,ro"
> when issued during an unlink of a 15GB file.  The disk I/O eventually
> completes, and drives go idle, but the system remains hung inside
> the remount,ro call.
> 
> Alt-sysrq-T was functioning, so I have some screen shots (.jpg) here:
> 
>  http://rtr.ca/remount_ro/
  Thanks for the traces.

> That's definitely a bug.
  Yes. We have a nice lock inversion there. ext3_remount() is called
with sb->s_lock held and waits for transaction to finish in
journal_lock_updates(). On the other hand ext3_orphan_del() is called
inside a transaction and tries to do lock_super()... Bad luck.

								Honza

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