On Wed, Apr 6, 2011 at 6:26 AM, Jan Kara <jack@xxxxxxx> 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@xxxxxxxxx>
>> Reported-by: Amir Goldstein <amir73il@xxxxxxxxxxxx>
>> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 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@xxxxxxx>"
> 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@xxxxxxx>
> line :)
>
> Honza
> --
> Jan Kara <jack@xxxxxxx>
> SUSE Labs, CR
>
--
Best Wishes
Yongqiang Yang
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