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Date: Mon, 21 Jun 2010 11:57:31 +1000 From: Dave Chinner <david@...morbit.com> To: Christoph Hellwig <hch@...radead.org> Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, viro@...IV.linux.org.uk, josef@...hat.com, jeffmerkey@...il.com Subject: Re: [PATCH 2/5] fsfreeze: emergency thaw will deadlock on s_umount 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@...morbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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