Re: [PATCH 2/5] fsfreeze: emergency thaw will deadlock on s_umount

[Dave Chinner <david@fromorbit.com>]

On Mon, Jun 14, 2010 at 11:20:11AM -0400, Christoph Hellwig wrote:
> On Thu, Jun 10, 2010 at 05:19:51PM +1000, Dave Chinner wrote:
> > The emergency thaw process uses iterate_super() which holds the
> > sb->s_umount lock in read mode. The current thaw_super() code takes
> > the sb->s_umount lock in write mode, hence leading to an instant
> > deadlock.
> > 
> > Pass the emergency state into the thaw_bdev/thaw_super code to avoid
> > taking the s_umount lock in this case. We are running under the bdev
> > freeze mutex, so this is still serialised against freeze despite
> > only having a read lock on the sb->s_umount. Hence it should be safe
> > to execute in this manner, especially given that emergency thaw is a
> > rarely executed "get-out-of-jail" feature.
> 
> This is correct as long as no one calls thaw_super directly, which
> is not the case currently.

This patch doesn't try to deal with the bdev/super mismatches; all
it does is prevent an obvious deadlock. Calling freeze/thaw_super
directly will serialise on the s_umount lock, calling
freeze/thaw_bdev() will serialise on the bdev freeze mutex, and if
we mix the two they'll serialise on the s_umount lock. So I think
with this patch serialisation will still occur correctly but avoid
the current deadlock.

I'll change the commit message to explain this better.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com