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

From: Dave Chinner
Date: Mon Jun 14 2010 - 19:22:22 EST

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.

Yeah, the idea of the first two patches is that they can be applied
to a current tree and backported and prevent the infinite loop or
deadlock. The problem of thaw_bdev/thaw_super is what the rest of
the patches are supposed to address.


Dave Chinner
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