From: Dave Chinner <david@fromorbit.com>
To: Benjamin Marzinski <bmarzins@redhat.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [RFC] fs: move freeze/unfreeze_fs hooks before freeze/thaw_super
Date: Tue, 23 Sep 2014 23:38:59 +1000 [thread overview]
Message-ID: <20140923133859.GR4267@dastard> (raw)
In-Reply-To: <1411438001-1919-1-git-send-email-bmarzins@redhat.com>
On Mon, Sep 22, 2014 at 09:06:41PM -0500, Benjamin Marzinski wrote:
> Currently, freezing a filesystem involves calling freeze_super, which locks
> sb->s_umount and then calls the fs-specific freeze_fs hook. This makes it
> hard for gfs2 (and potentially other cluster filesystems) to use the vfs
> freezing code to do freezes on all the cluster nodes.
>
> In order to communicate that a freeze has been requested, and to make sure
> that only one node is trying to freeze at a time, gfs2 uses a glock
> (sd_freeze_gl). The problem is that there is no hook for gfs2 to acquire
> this lock before calling freeze_super. This means that two nodes can
> attempt to freeze the filesystem by both calling freeze_super, acquiring
> the sb->s_umount lock, and then attempting to grab the cluster glock
> sd_freeze_gl. Only one will succeed, and the other will be stuck in
> freeze_super, making it impossible to finish freezing the node.
>
> To solve this problem, this patch pushes the freeze/unfreeze_fs hooks to
> before freeze/thaw_super, and makes freeze/thaw_super take a callback
> function to execute any fs specific code that needs to be done while
> s_umount is held. This also means that every filesystem that implements
> freeze/unfreeze_fs must call freeze/thaw_super from that function.
Wouldn't just adding a ->prepare_freeze/->prepare_thaw method pair
and adding gfs2 cluster locking to those new methods be better?
That way no other filesystem code needs to change at all...
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2014-09-23 13:39 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-23 2:06 [RFC] fs: move freeze/unfreeze_fs hooks before freeze/thaw_super Benjamin Marzinski
2014-09-23 13:38 ` Dave Chinner [this message]
2014-09-23 13:24 ` Benjamin Marzinski
2014-09-23 20:50 ` Dave Chinner
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