linux-kernel - Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync

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Message-ID: <CALTww28UapJnK+Xfx7O9uEd5ZH2E7ufPT_7pKY6YYuzTZ0Fbdw@mail.gmail.com>
Date:   Wed, 14 Jun 2023 15:57:53 +0800
From:   Xiao Ni <xni@...hat.com>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     guoqing.jiang@...ux.dev, agk@...hat.com, snitzer@...nel.org,
        dm-devel@...hat.com, song@...nel.org, linux-raid@...r.kernel.org,
        yangerkun@...wei.com, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync_thread()
 to fix deadlock

On Wed, Jun 14, 2023 at 3:38 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> Hi,
>
> 在 2023/06/14 15:12, Xiao Ni 写道:
> > On Wed, Jun 14, 2023 at 10:04 AM Yu Kuai <yukuai1@...weicloud.com> 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?

>
> 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.

>
> > 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?

Regards
Xiao

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