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

[Yu Kuai]

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().

> > > 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.

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