Date: Mon, 14 Jun 2010 11:20:11 -0400
From: Christoph Hellwig <hch@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
       viro@ZenIV.linux.org.uk, josef@redhat.com, jeffmerkey@gmail.com
Subject: Re: [PATCH 2/5] fsfreeze: emergency thaw will deadlock on s_umount
Message-ID: <20100614152011.GB32354@infradead.org>
References: <1276154395-24766-1-git-send-email-david@fromorbit.com>
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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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