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

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Message-ID: <20061107234951.GD30653@agk.surrey.redhat.com>
Date:	Tue, 7 Nov 2006 23:49:51 +0000
From:	Alasdair G Kergon <agk@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	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 12:05:49AM +0100, Rafael J. Wysocki wrote:
> But freeze_bdev() is supposed to return the result of get_super(bdev)
> _unconditionally_.  Moreover, in its current form freeze_bdev() _cannot_
> _fail_, so I don't see how this change doesn't break any existing code.
> For example freeze_filesystems() (recently added to -mm) will be broken
> if the down_trylock() is unsuccessful.

I hadn't noticed that -mm patch.  I'll take a look.  Up to now, device-mapper
(via dmsetup) and xfs (via xfs_freeze, which dates from before device-mapper
handled this automatically) were the only users.  Only one freeze should be
issued at once.  A freeze is a temporary thing, normally used while creating a
snapshot.  (One problem we still have is lots of old documentation on the web
advising people to run xfs_freeze before creating device-mapper snapshots.)

You're right that the down_trylock idea is more trouble than it's worth and
should be scrapped.

Alasdair
-- 
agk@...hat.com
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