Re: [PATCH] jbd2: assign different lock_class_key for different filesystem

Tetsuo Handa
2025-10-21T11:17:19Z

On 2025/10/20 21:55, Jan Kara wrote:
> On Mon 20-10-25 20:28:54, Tetsuo Handa wrote:
>> On 2025/10/20 18:31, Jan Kara wrote:
>>> On Sun 19-10-25 19:52:43, Tetsuo Handa wrote:
>>>> syzbot is reporting possibility of deadlock due to sharing lock_class_key
>>>> for jbd2_handle across ext4 and ocfs2. But one disk partition can't have
>>>> two filesystems at the same time, and how locks in journal_t interacts
>>>> with other filesystem specific locks can vary depending on filesystems.
>>>> Therefore, lockdep should treat locks in journal_t different locks if
>>>> the filesystem which allocated the journal_t differs.
>>>
>>> Thanks for debugging this. I agree with the idea of your solution but the
>>> implementation is just ugly. Just let the filesystem pass the lockdep key
>>> into jbd2_journal_init_dev() / jbd2_journal_init_inode() and make ext4 and
>>> ocfs2 functions initializing the journal each have its own lock_class_key
>>> declared and pass it to jbd2 functions. That way changes are much simpler
>>> and also jbd2 doesn't have to be aware about which filesystems are using
>>> it.
>>
>> Well, do you mean something like below diff? If we can assume that only ext4
>> and ocfs2 are using jbd2, the diff becomes smaller...
>
> Yes, something like this. In fact, I think we could get away with just
> jbd2_trans_commit_key. There's definitely no need for j_revoke_lock,
> j_list_lock, j_history_lock, j_state_lock, j_abort_mutex keys as these are
> internal to jbd2. j_checkpoint_mutex and j_barrier do wrap around some
> filesystem code so maybe we'll need to specify keys for them but I'd start
> with just jbd2_trans_commit_key and wait whether syzbot manages to trigger
> another false positive report with that.

I tried https://syzkaller.appspot.com/x/patch.diff?x=11b4dde2580000 .
But I think https://syzkaller.appspot.com/x/patch.diff?x=1644c3cd980000
pattern which all mutex_init_with_key() users follow seems more simpler
and easier to apply. What do you think?