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

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Message-ID: <20110406225922.GI31057@dastard>
Date:	Thu, 7 Apr 2011 08:59:23 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jan Kara <jack@...e.cz>,
	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 Wed, Apr 06, 2011 at 09:44:28AM -0400, Christoph Hellwig wrote:
> On Wed, Apr 06, 2011 at 09:21:35PM +1000, Dave Chinner wrote:
> > Sure, but the question must be asked - why is ext3/4 even starting a
> > transaction on a clean filesystem during sync? A frozen filesystem,
> > by definition, is a clean filesytem, and therefore sync calls of any
> > kind should not be trying to write to the FS or start transactions.
> > XFS does this just fine, so I'd consider such behaviour on a frozen
> > filesystem a bug in ext3/4...
> 
> XFS does have one special case for this.  When writing the dummy log
> record at the end of the freeze process we use _xfs_alloc_trans to
> bypass the frozen filesystem check as we have to write out this record
> when the filesystem already is frozen.  But that's after the main
> sync with its normal transactions.

Right, that is a special case in the _freeze process_ (i.e. before
we've declared the FS frozen), not a normal operation on a frozen
filesystem.

If you want to list exceptions (i.e. where we explicitly avoid
writes to frozen fs), look for xfs_fs_writeable(), which stops
various write operations from proceeding when the fs is either
frozen, read-only or shut down.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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