Date: Mon, 21 Jun 2010 03:47:56 -0400
From: Christoph Hellwig <hch@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>, 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: <20100621074756.GA2201@infradead.org>
References: <1276154395-24766-1-git-send-email-david@fromorbit.com>
 <1276154395-24766-3-git-send-email-david@fromorbit.com>
 <20100614152011.GB32354@infradead.org>
 <20100621015731.GJ6590@dastard>
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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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