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 14:20:55 +0200
From:	Jan Kara <jack@...e.cz>
To:	Surbhi Palande <surbhi.palande@...ntu.com>
Cc:	Jan Kara <jack@...e.cz>, 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 02-05-11 14:27:51, Surbhi Palande wrote:
> On 05/02/2011 01:56 PM, Jan Kara wrote:
> >On Mon 02-05-11 12:07:59, Surbhi Palande wrote:
> >>On 04/06/2011 02:21 PM, Dave Chinner wrote:
> >>>On Wed, Apr 06, 2011 at 08:18:56AM +0200, Jan Kara wrote:
> >>>>On Wed 06-04-11 15:40:05, Dave Chinner wrote:
> >>>>>On Fri, Apr 01, 2011 at 04:08:56PM +0200, Jan Kara wrote:
> >>>>>>On Fri 01-04-11 10:40:50, Dave Chinner wrote:
> >>>>>>>If you don't allow the page to be dirtied in the fist place, then
> >>>>>>>nothing needs to be done to the writeback path because there is
> >>>>>>>nothing dirty for it to write back.
> >>>>>>   Sure but that's only the problem he was able to hit. But generally,
> >>>>>>there's a problem with needing s_umount for unfreezing because it isn't
> >>>>>>clear there aren't other code paths which can block with s_umount held
> >>>>>>waiting for fs to get unfrozen. And these code paths would cause the same
> >>>>>>deadlock. That's why I chose to get rid of s_umount during thawing.
> >>>>>Holding the s_umount lock while checking if frozen and sleeping
> >>>>>is essentially an ABBA lock inversion bug that can bite in many more
> >>>>>places that just thawing the filesystem.  Any where this is done should
> >>>>>be fixed, so I don't think just removing the s_umount lock from the thaw
> >>>>>path is sufficient to avoid problems.
> >>>>   That's easily said but hard to do - any transaction start in ext3/4 may
> >>>>block on filesystem being frozen (this seems to be similar for XFS as I'm
> >>>>looking into the code) and transaction start traditionally nests inside
> >>>>s_umount (and basically there's no way around that since sync() calls your
> >>>>fs code with s_umount held).
> >>>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...
> >>I had a look at the xfs code for seeing how this is done.
> >>xfs_file_aio_write()
> >>   xfs_wait_for_freeze()
> >>     vfs_check_frozen()
> >>So xfs_file_aio_write() writes to buffers when the FS is not frozen.
> >>
> >>Now, I want to know what stops the following scenario from happening:
> >>--------------------
> >>xfs_file_aio_write()
> >>   xfs_wait_for_freeze()
> >>     vfs_check_frozen()
> >>At this point F.S was not frozen, so the next instruction in the
> >>xfs_file_aio_write() will be executed next.
> >>However at this point (i.e after checking if F.S is frozen) the
> >>write process gets pre-empted and say the _freeze_ process gets
> >>control.
> >>
> >>Now the F.S freezes and the write process gets the control back. And
> >>so we end up writing to the page cache when the F.S is frozen.
> >>--------------------
> >>
> >>Can anyone please enlighten me on how&  why this premption is _not_
> >>possible?
> Thanks for your reply.
> >   XFS works similarly as ext4 in this regard I believe. They have the log
> >frozen in xfs_freeze() so if the race you describe above happens, either
> >the writing process gets caught waiting for log to unfreeze
> Agreed.
> >  or it manages
> >to start a transaction and then freezing process waits for transaction to
> >finish before it can proceed with freezing. I'm not sure why is there the
> >check in xfs_file_aio_write()...
> >
> >			
> I am sorry, but I don't understand how this will happen - i.e I
> can't understand what stops freeze_super() (or ext4_freeze) from
> freezing a superblock (as the write process stopped just before
> writing anything for this transaction and has not taken any locks?)
  So ext4_freeze() does
jbd2_journal_lock_updates(journal)
  which waits for all running transactions to finish and updates
j_barrier_count which stops any news ones from proceeding (check
function start_this_handle()).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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