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

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Date:	Thu, 31 Mar 2011 10:25:30 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.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 3/31/11 9:36 AM, Yongqiang Yang wrote:
> On Thu, Mar 31, 2011 at 10:04 PM, Eric Sandeen <sandeen@...hat.com> wrote:
>> On 3/31/11 3:37 AM, Yongqiang Yang wrote:
>>
>>> in ext3, ext3_freeze() prevents journal from being updated by
>>> lock_journal_updates(), ext3_unfreeze() allow journal to be updated by
>>> unlock_journal_updates().
>>>
>>> in ext4, however, before ext4_freeze() returns, it unlock journal, and
>>> ext4 prevents journal from being updated by s_frozen. s_frozen is in
>>> an upper layer, so it is out control of ext4 and deadlock is easy to
>>> happen.
>>>
>>> Could someone explain why ext4 does like above but not follow ext3?
>>>
>>> Yongqiang.
>>
>> That was me, I think ...
> 
> Thank you, Eric.
> 
> I think ext4_journal_start() should check if current thread has an
> active handle before  vfs_check_frozen(), if so, current handle will
> be returned. Thus, we can avoid deadlocks.
> 
> Do you agree with me?  If I am right, I will send a patch.

If you have a testcase to test it with, sure.  plus a patch would help me know for sure what you propose :)

Sorry for breaking it (if I did!)  But holding a mutex and returning to userspace was pretty bad, too :(

Thanks,
-Eric

>>
>> commit 6b0310fbf087ad6e9e3b8392adca97cd77184084
>> Author: Eric Sandeen <sandeen@...hat.com>
>> Date:   Sun May 16 02:00:00 2010 -0400
>>
>>    ext4: don't return to userspace after freezing the fs with a mutex held
>>
>>    ext4_freeze() used jbd2_journal_lock_updates() which takes
>>    the j_barrier mutex, and then returns to userspace.  The
>>    kernel does not like this:
>>
>>    ================================================
>>    [ 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
>>
>>    Use vfs_check_frozen() added to ext4_journal_start_sb() and
>>    ext4_force_commit() instead.
>>
>>    Addresses-Red-Hat-Bugzilla: #568503
>>
>>
>>    Signed-off-by: Eric Sandeen <sandeen@...hat.com>
>>    Signed-off-by: "Theodore Ts'o" <tytso@....edu>
>>
> 
> 
> 

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