On 3/31/11 9:36 AM, Yongqiang Yang wrote:
> On Thu, Mar 31, 2011 at 10:04 PM, Eric Sandeen <sandeen@xxxxxxxxxx> 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@xxxxxxxxxx>
>> 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@xxxxxxxxxx>
>> Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
>>
>
>
>
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