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

[Xiao Ni]

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?

Regards
Xiao
>
> Thanks,
> Kuai
> >
> > Regards
> > Xiao
> >
> >>
> >> Thanks,
> >> Kuai
> >>>
> >>> Then admin2 echos idle > sync_action, sync_seq is still 1
> >>>
> >>> Regards
> >>> Xiao
> >>>
> >>>>
> >>>> Thanks,
> >>>> Kuai
> >>>>
> >>>
> >>> .
> >>>
> >>
> >
> > .
> >
>