Re: [RFC] fs: move freeze/unfreeze_fs hooks before freeze/thaw_super

[Dave Chinner <david@fromorbit.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