Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync_thread() to fix deadlock

[Xiao Ni]

On Thu, Jun 15, 2023 at 4:01 PM Xiao Ni <xni@xxxxxxxxxx> wrote:
>
> On Thu, Jun 15, 2023 at 9:29 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > 在 2023/06/14 17:08, Xiao Ni 写道:
> > > On Wed, Jun 14, 2023 at 4:29 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > >>
> > >> Hi,
> > >>
> > >> 在 2023/06/14 15:57, Xiao Ni 写道:
> > >>> On Wed, Jun 14, 2023 at 3:38 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > >>>>
> > >>>> Hi,
> > >>>>
> > >>>> 在 2023/06/14 15:12, Xiao Ni 写道:
> > >>>>> On Wed, Jun 14, 2023 at 10:04 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > >>>>>>
> > >>>>>> Hi,
> > >>>>>>
> > >>>>>> 在 2023/06/14 9:48, Yu Kuai 写道:
> > >>>>>>
> > >>>>>>
> > >>>>>>>>
> > >>>>>>>> In the patch, sync_seq is added in md_reap_sync_thread. In
> > >>>>>>>> idle_sync_thread, if sync_seq isn't equal
> > >>>>>>>>
> > >>>>>>>> mddev->sync_seq, it should mean there is someone that stops the sync
> > >>>>>>>> thread already, right? Why do
> > >>>>>>>>
> > >>>>>>>> you say 'new started sync thread' here?
> > >>>>>>
> > >>>>>> If someone stops the sync thread, and new sync thread is not started,
> > >>>>>> then this sync_seq won't make a difference, above wait_event() will not
> > >>>>>> wait because !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) will pass.
> > >>>>>> So 'sync_seq' is only used when the old sync thread stops and new sync
> > >>>>>> thread starts, add 'sync_seq' will bypass this case.
> > >>>>>
> > >>>>> Hi
> > >>>>>
> > >>>>> If a new sync thread starts, why can sync_seq be different? sync_seq
> > >>>>> is only added in md_reap_sync_thread. And when a new sync request
> > >>>>> starts, it can't stop the sync request again?
> > >>>>>
> > >>>>> Af first, the sync_seq is 0
> > >>>>>
> > >>>>> admin1
> > >>>>> echo idle > sync_action
> > >>>>> idle_sync_thread(sync_seq is 1)
> > >>>>
> > >>>> Wait, I'm confused here, how can sync_seq to be 1 here? I suppose you
> > >>>> mean that there is a sync_thread just finished?
> > >>>
> > >>> Hi Kuai
> > >>>
> > >>> Yes. Because idle_sync_thread needs to wait until md_reap_sync_thread
> > >>> finishes. And md_reap_sync_thread adds sync_seq. Do I understand your
> > >>> patch right?
> > >>
> > >> Yes, noted that idle_sync_thread() will only wait if MD_RECOVERY_RUNNING
> > >> is set.
> > >>
> > >>>
> > >>>>
> > >>>> Then the problem is that idle_sync_thread() read sync_seq after the old
> > >>>> sync_thread is done, and new sync_thread start before wait_event() is
> > >>>> called, should we wait for this new sync_thread?
> > >>>>
> > >>>> My answer here is that we should, but I'm also ok to not wait this new
> > >>>> sync_thread, I don't think this behaviour matters. The key point here
> > >>>> is that once wait_event() is called from idle_sync_thread(), this
> > >>>> wait_event() should not wait for new sync_thread...
> > >>>
> > >>> I think we should wait. If we don't wait for it, there is a problem.
> > >>> One person echos idle to sync_action and it doesn't work sometimes.
> > >>> It's a strange thing.
> > >>>
> > >>
> > >> Ok. I'll add new comment to emphasize that idle_sync_thread() won't wait
> > >> for new sync_thread that is started after wait_event().
> > >
> > > I suggest removing this function. Without this change, it's more
> > > simple and it can work well without problem. The people that echo idle
> > > to sync_action needs to wait until the sync action finishes. The code
> > > semantic is clear and simple.
> > >>
> > >>>>
> > >>>>> echo resync > sync_action (new sync)
> > >>>>
> > >>>> If this is behind "echo idle > sync_action", idle_sync_thread should not
> > >>>> see that MD_RECOVERY_RUNNING is set and wait_event() won't wait at all.
> > >>>
> > >>> `echo resync > sync_action` can't change the sync_seq. So 'echo idle >
> > >>> sync_action' still waits until MD_RECOVERY_RUNNING is cleared?
> > >>
> > >> This is not accurate, if `echo resync > sync_action` triggers a new
> > >> sync_thread, then sync_seq is updated when this sync_thread is done,
> > >> during this period, MD_RECOVERY_RUNNING is still set, so `echo idle
> > >>   >sync_action` will wait for sync_thread to be done.
> > >
> > > I can understand your comment, but sorry, I still can't get how
> > > sync_seq works. Could you give a specific case that explains how it
> > > works?
> >
> > Ok, the problem is that echo ilde is supposed to interrupt sync_thread
> > and stop sync_thread quickly. Now that we don't hold mutex here, we
> > can't prevent new sync_thread to start. For exapmle:
> >
> > 1) a sync_thread A is runing, MD_RECOVERY_RUNNING is set;
> >
> > 2) echo idle, A will be interrupted, mutex is not hold and
> > idle_sync_thread() is waiting for MD_RECOVERY_RUNNING to be cleared.
> >
> > 3) A is interrupted, it'll clear MD_RECOVERY_RUNNING and try to wakeup
> > idle_sync_thread(), however, before idle_sync_thread() is woken, A can
> > be done and a new sync_thread B can be started, and MD_RECOVERY_RUNNING
> > will be set again.
> >
> > 4) idle_sync_thread() finially wake up, however, MD_RECOVERY_RUNNING is
> > set and it will still waiting. And this time B won't be interrupted.
>
> Thanks for the example. I can understand the usage of it. It's the
> side effect that removes the mutex protection for idle_sync_thread.
>
> There is a problem. New sync thread is started in md_check_recovery.
> After your patch, md_reap_sync_thread is called in md_check_recovery
> too. So it looks like they can't happen at the same time?

After thinking a while, there is still a race possibility.

md_reap_sync_thread is called in pers deamon (e.g. raid10d ->
md_check_recovery) and md_check_recovery returns. Before
idle_sync_thread is woken, the new sync thread can be started in
md_check_recovery again.

But it's really strange, when one people echo idle to sync_action.
It's better to add some messages to notify the users that they need to
echo idle to sync_action again to have a try. Is there a way that
md_reap_sync_thread can wait idle_sync_thread?

Regards
Xiao
>
> Regards
> Xiao
>
> >
> > Thanks,
> > Kuai
> >
> > --
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