Messages in this thread | | | Date | Mon, 21 Jun 2010 11:57:31 +1000 | From | Dave Chinner <> | Subject | Re: [PATCH 2/5] fsfreeze: emergency thaw will deadlock on s_umount |
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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
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