linux-ext4 - Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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Date:	Mon, 2 May 2011 12:38:49 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Surbhi Palande <surbhi.palande@...ntu.com>,
	Dave Chinner <david@...morbit.com>,
	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>,
	Ted Ts'o <tytso@....edu>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to
 a deadlock

On Mon, May 02, 2011 at 06:23:34PM +0200, Jan Kara wrote:
>   But why should it block when xfs_wait_for_freeze() gets called before
> freeze_super() gets called? The other thread calls freeze_super() just
> after xfs_wait_for_freeze() in thread 1 and before _xfs_trans_alloc() gets
> called.  Or am I missing some serialization somewhere?

Oh, now I get the race window you mean.  It's the single instruction
window between doing the frozen check and incrementing m_active_trans.

Yes, that one looks real, although very unlikely to hit.  Could be fixed
relatively easily by moving the check after the increment.
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