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In-Reply-To: <04700f85-62a2-1dbd-f330-80f9a13b7d2e@huaweicloud.com>
From:   Xiao Ni <xni@redhat.com>
Date:   Thu, 15 Jun 2023 16:01:17 +0800
Message-ID: <CALTww2-Wr8UbNFaLOyYv5Syh5q4J+hzRuo8Eakj_nOW+P4Cx7A@mail.gmail.com>
Subject: Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync_thread()
 to fix deadlock
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     yi.zhang@huawei.com, yangerkun@huawei.com, snitzer@kernel.org,
        linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org,
        song@kernel.org, dm-devel@redhat.com, guoqing.jiang@linux.dev,
        "yukuai (C)" <yukuai3@huawei.com>, agk@redhat.com
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On Thu, Jun 15, 2023 at 9:29=E2=80=AFAM Yu Kuai <yukuai1@huaweicloud.com> w=
rote:
>
> Hi,
>
> =E5=9C=A8 2023/06/14 17:08, Xiao Ni =E5=86=99=E9=81=93:
> > On Wed, Jun 14, 2023 at 4:29=E2=80=AFPM Yu Kuai <yukuai1@huaweicloud.co=
m> wrote:
> >>
> >> Hi,
> >>
> >> =E5=9C=A8 2023/06/14 15:57, Xiao Ni =E5=86=99=E9=81=93:
> >>> On Wed, Jun 14, 2023 at 3:38=E2=80=AFPM Yu Kuai <yukuai1@huaweicloud.=
com> wrote:
> >>>>
> >>>> Hi,
> >>>>
> >>>> =E5=9C=A8 2023/06/14 15:12, Xiao Ni =E5=86=99=E9=81=93:
> >>>>> On Wed, Jun 14, 2023 at 10:04=E2=80=AFAM Yu Kuai <yukuai1@huaweiclo=
ud.com> wrote:
> >>>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> =E5=9C=A8 2023/06/14 9:48, Yu Kuai =E5=86=99=E9=81=93:
> >>>>>>
> >>>>>>
> >>>>>>>>
> >>>>>>>> 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 start=
ed,
> >>>>>> then this sync_seq won't make a difference, above wait_event() wil=
l 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_se=
q
> >>>>> 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 yo=
u
> >>>> 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_RUNNI=
NG
> >> 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() i=
s
> >>>> 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 n=
ew
> >>>> sync_thread, I don't think this behaviour matters. The key point her=
e
> >>>> 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 wa=
it
> >> 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 a=
ll.
> >>>
> >>> `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 for the example. I can understand the usage of it. It's the
side effect that removes the mutex protection for idle_sync_thread.

There is a problem. New sync thread is started in md_check_recovery.
After your patch, md_reap_sync_thread is called in md_check_recovery
too. So it looks like they can't happen at the same time?

Regards
Xiao

>
> Thanks,
> Kuai
>
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