From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to
 a deadlock
Date: Mon, 2 May 2011 10:41:55 -0400
Message-ID: <20110502144155.GA8045@infradead.org>
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Cc: Christoph Hellwig <hch@infradead.org>,
	Surbhi Palande <surbhi.palande@ubuntu.com>,
	Dave Chinner <david@fromorbit.com>,
	Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>,
	Ted Ts'o <tytso@mit.edu>,
	Masayoshi MIZUMA <m.mizuma@jp.fujitsu.com>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org
To: Jan Kara <jack@suse.cz>
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On Mon, May 02, 2011 at 04:20:55PM +0200, Jan Kara wrote:
>   Hmm, but what prevents the following race?
> 
>   Thread 1					Thread 2
> ..
> xfs_trans_alloc()
>   xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
> 						freeze_super()

                                                  sb->s_frozen = SB_FREEZE_TRANS;

> 						  ...
> 						  xfs_fs_freeze()
> 						    ...
> 						    xfs_quiesce_attr()

						      waits for all active
						      transactions

> 						    ...

   xfs_trans_alloc
     -> blocks in xfs_wait_for_freeze
     (thus doesn't get to _xfs_trans_alloc)

>   _xfs_trans_alloc()
>     atomic_inc(&mp->m_active_trans);
>     ... goes on modifying the filesystem
> 
>   It seems to be a similar problem as in ext4 - the atomic_inc() and
> vfs_check_frozen() are in the wrong order...

I can't see the problem in this scheme.  Note that we want
_xfs_trans_alloc to be able to create a transaction for
xfs_fs_log_dummy, so that we can write the dummy log record after
freezing out all other transactions, so that one is special cased
and doesn't do the xfs_wait_for_freeze.