Date:	Tue, 03 May 2011 10:27:00 +0300
From:	Surbhi Palande <surbhi.palande@...ntu.com>
To:	Eric Sandeen <sandeen@...hat.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,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

On 05/02/2011 05:04 PM, Eric Sandeen wrote:
> On 5/2/11 8:22 AM, Surbhi Palande wrote:
>> On 05/02/2011 04:16 PM, Jan Kara wrote:
>>> On Mon 02-05-11 15:30:23, Surbhi Palande wrote:
>>>> On 05/02/2011 03:20 PM, Jan Kara wrote:
>>>>> 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()).
>>>>>
>>>> Yes, but ext4_freeze() also calls
>>>> jbd2_journal_unlock_updates(journal) which decrements the
>>>> j_barrier_count (which was previously updated/incremented in
>>>> jbd2_journal_lock_updates) ? before it returns. So after this call a
>>>> new transaction/handle can be accepted/started.
>>>>
>>>> A comment in ext4_freeze() says:
>>>> /* we rely on s_frozen to stop further updates */
>>>> (before calling jbd2_journal_unlock_updates())
>>>     Ah, drat, you're right. I've missed this other part. It's the problem
>>> that if you expect to see something, you'll see it regardless of the real
>>> code ;).
>>>
>>> The fact is we do vfs_check_frozen() in ext4_journal_start_sb() but indeed
>>> it's still racy (although the race window is relatively small) because the
>>> filesystem can become frozen the instant after we check vfs_check_frozen().
>>> Commit 6b0310fb broke it for ext4.
>>>
>>> I guess the code was mostly copied from XFS which seems to have the same
>>> problem in xfs_trans_alloc() since the git history beginning. I see two
>>> ways to fix this - either fix ext4/xfs to check s_frozen after starting
>>> a transaction and if the filesystem is being frozen, we stop the
>>> transaction, wait for fs to get unfrozen, and restart. Another option is
>>> to create an analogous logic using a atomic counter of write ops in vfs
>>> that could be used by all filesystems. We'd just have to replace
>>> vfs_check_frozen() with vfs_start_write() and add vfs_stop_write() at
>>> appropriate places...
>> How about calling  jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
>> from ext4_unfreeze()?
> we used to have that, but holding it locked until then means we exit the kernel
> with a mutex held, which is pretty icky.
>
>      ================================================
>      [ BUG: lock held when returning to user space! ]
>      ------------------------------------------------
>      lvcreate/1075 is leaving the kernel with locks still held!
>      1 lock held by lvcreate/1075:
>       #0:  (&journal->j_barrier){+.+...}, at: [<ffffffff811c6214>]
>      jbd2_journal_lock_updates+0xe1/0xf0
>
>
> -Eric
Should this not be reverted? I think that its a lot easier to stop a 
transaction between a freeze and a thaw that way! If you agree, can I 
send a patch for the same?

Thanks!

Warm Regards,
Surbhi.


>> So that indeed no transactions can be started before unfreeze is called.
>>
>> This has another advantage, that it rightfully does not let you update the access time when the F.S is frozen (touch_atime called from a read path when the F.S is frozen) Otherwise we also need to fix this path.
>>
>> Warm Regards,
>> Surbhi.
>>
>>> Dave, Christoph, any opinions on this?
>>>                                  Honza
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