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.
>
> 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>
>
--
Best Wishes
Yongqiang Yang
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