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

[Yu Kuai]

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,
Kuai