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

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Message-Id: <20070607124507.79d8da54.akpm@linux-foundation.org>
Date:	Thu, 7 Jun 2007 12:45:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Mark Lord <lkml@....ca>, 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

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.
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