linux-kernel - Re: [PATCH 2.6.19 5/5] fs: freeze

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Message-ID: <20061108082722.GH8394166@melbourne.sgi.com>
Date:	Wed, 8 Nov 2006 19:27:22 +1100
From:	David Chinner <dgc@....com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	Alasdair G Kergon <agk@...hat.com>,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	Ingo Molnar <mingo@...e.hu>,
	Srinivasa DS <srinivasa@...ibm.com>
Subject: Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

On Wed, Nov 08, 2006 at 12:01:09AM +0000, Alasdair G Kergon wrote:
> On Wed, Nov 08, 2006 at 12:42:02AM +0100, Rafael J. Wysocki wrote:
> > On Wednesday, 8 November 2006 00:18, Eric Sandeen wrote:
> > > But, how is a stampede of fs-freezers -supposed- to work?  I could
> > > imagine something like a freezer count, and the filesystem is only
> > > unfrozen after everyone has thawed?  Or should only one freezer be
> > > active at a time... which is what we have now I guess.
> > I think it shouldn't be possible to freeze an fs more than once.
> 
> In device-mapper today, the only way to get more than one freeze on the
> same device is to use xfs and issue xfs_freeze before creating an lvm snapshot
> (or issuing the dmsetup equivalent), and at the moment we tell people not to do
> that any more.

But it's trivial to detect this condition - if (sb->s_frozen != SB_UNFROZEN)
then the filesystem is already frozen and you shouldn't try to freeze
it again. It's simple to do, and the whole problem then just goes away....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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