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

From: Christoph Hellwig
Date: Mon Jun 14 2010 - 11:20:19 EST

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.

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