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Date:	Wed, 6 Apr 2011 08:52:40 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"Ted Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v1] ext4:Allow an active handle to be started when freezing.

On Wed, Apr 6, 2011 at 6:26 AM, Jan Kara <jack@...e.cz> wrote:
> On Tue 05-04-11 15:47:25, Yongqiang Yang wrote:
>> v0->v1:
>>   check s_frozen in nojournal case only if there is no an active handle.
>>   get reference to an active handle by jbd2_journal_start().
>>
>> ext4_journal_start_sb() should not prevent an active handle from being
>> started due to s_frozen.  Otherwise, deadlock is easy to happen, below
>> is a situation.
>>
>> ================================================
>>      freeze         |       truncate
>> ================================================
>>                     |  ext4_ext_truncate()
>>     freeze_super()  |   starts a handle
>>     sets s_frozen   |
>>                     |  ext4_ext_truncate()
>>                     |  holds i_data_sem
>>   ext4_freeze()     |
>>   waits for updates |
>>                     |  ext4_free_blocks()
>>                     |  calls dquot_free_block()
>>                     |
>>                     |  dquot_free_blocks()
>>                     |  calls ext4_dirty_inode()
>>                     |
>>                     |  ext4_dirty_inode()
>>                     |  trys to start an active
>>                     |  handle
>>                     |
>>                     |  block due to s_frozen
>> ================================================
>>
>> Signed-off-by: Yongqiang Yang <xiaoqiangnk@...il.com>
>> Reported-by: Amir Goldstein <amir73il@...rs.sf.net>
>> Reviewed-by: Jan Kara <jack@...e.cz>
>  The usual workflow is that you add 'Reviewed-by' tag to the patch only
> after you send a version of a patch the person agrees to and explicitely
> says you can add a tag - for example I write something like "You can add
> Reviewed-by: Jan Kara <jack@...e.cz>"
> when I agree to the tag being added.
I see.  Thank you.
>
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index ccfa686..53920bd 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -242,27 +242,45 @@ static void ext4_put_nojournal(handle_t *handle)
>>   * journal_end calls result in the superblock being marked dirty, so
>>   * that sync() will call the filesystem's write_super callback if
>>   * appropriate.
>> + *
>> + * To avoid j_barrier hold in userspace when a user calls freeze(),
>> + * ext4 prevents a new handle from being started by s_frozen, which
>> + * is in an upper layer.
>>   */
>>  handle_t *ext4_journal_start_sb(struct super_block *sb, int nblocks)
>>  {
>>       journal_t *journal;
>> +     handle_t  *handle;
>>
>>       if (sb->s_flags & MS_RDONLY)
>>               return ERR_PTR(-EROFS);
>>
>> -     vfs_check_frozen(sb, SB_FREEZE_TRANS);
>> -     /* Special case here: if the journal has aborted behind our
>> -      * backs (eg. EIO in the commit thread), then we still need to
>> -      * take the FS itself readonly cleanly. */
>>       journal = EXT4_SB(sb)->s_journal;
>> -     if (journal) {
>> -             if (is_journal_aborted(journal)) {
>> -                     ext4_abort(sb, "Detected aborted journal");
>> -                     return ERR_PTR(-EROFS);
>> -             }
>> -             return jbd2_journal_start(journal, nblocks);
>> +     handle = ext4_journal_current_handle();
>> +
>> +     /*
>> +      * Before vfs_check_frozen(), the current handle should be allowed
>> +      * to finish, otherwise deadlock would happen when the filesystem
>> +      * is freezed && active handles are not stopped.
>> +      */
>> +     if (!journal) {
>> +             if (!handle)
>> +                     vfs_check_frozen(sb, SB_FREEZE_TRANS);
>> +             return ext4_get_nojournal();
>>       }
>> -     return ext4_get_nojournal();
>> +
>> +     if (!handle)
>> +             vfs_check_frozen(sb, SB_FREEZE_TRANS);
>  This test is the same for 'nojournal' case so you can move it before the
> !journal test. Otherwise the patch looks OK to me. So now you can add
> Reviewed-by: Jan Kara <jack@...e.cz>
> line :)
>
>                                                                        Honza
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
>



-- 
Best Wishes
Yongqiang Yang
--
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