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

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Date:	Mon, 02 May 2011 14:27:51 +0300
From:	Surbhi Palande <surbhi.palande@...ntu.com>
To:	Jan Kara <jack@...e.cz>
CC:	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 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?)

Thanks!

Warm Regards,
Surbhi.
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