linux-kernel - Re: [PATCH 2/5] fsfreeze: emergency thaw will deadlock on s

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Message-ID: <20100614152011.GB32354@infradead.org>
Date:	Mon, 14 Jun 2010 11:20:11 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>
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 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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