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

[Yu Kuai]

Hi,

在 2023/06/14 15:12, 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?

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...

> 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.

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