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

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Date:	Mon, 21 Jun 2010 03:47:56 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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 21, 2010 at 11:57:31AM +1000, Dave Chinner wrote:
> 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.

I don;t think the explanation alone is enough.

Right now thaw_super itself is only serialized by exclusive shared
s_umount.   thaw_bdev it also serialized by bd_fsfreeze_mutex, but
there are callers of thaw_super that do not go through thaw_bdev, so
our locking is not enough here.

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