linux-kernel - Re: md_raid: mdX_raid6 looping after sync

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Message-Id: <20230822211627.1389410-1-dragan@stancevic.com>
Date:   Tue, 22 Aug 2023 16:16:27 -0500
From:   Dragan Stancevic <dragan@...ncevic.com>
To:     song@...nel.org
Cc:     buczek@...gen.mpg.de, guoqing.jiang@...ux.dev,
        it+raid@...gen.mpg.de, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org, msmith626@...il.com,
        yukuai1@...weicloud.com
Subject: Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle" transition

On Tue, 3/28/23 17:01 Song Liu wrote:
> On Thu, Mar 16, 2023 at 8:25=E2=80=AFAM Marc Smith <msmith626@...il.com> 
> wr=
> ote:
>  >
>  > On Tue, Mar 14, 2023 at 10:45=E2=80=AFAM Marc Smith 
> <msmith626@...il.com>=
>   wrote:
>  > >
>  > > On Tue, Mar 14, 2023 at 9:55=E2=80=AFAM Guoqing Jiang 
> <guoqing.jiang@li=
> nux.dev> wrote:
>  > > >
>  > > >
>  > > >
>  > > > On 3/14/23 21:25, Marc Smith wrote:
>  > > > > On Mon, Feb 8, 2021 at 7:49=E2=80=AFPM Guoqing Jiang
>  > > > > <guoqing.jiang@...ud.ionos.com> wrote:
>  > > > >> Hi Donald,
>  > > > >>
>  > > > >> On 2/8/21 19:41, Donald Buczek wrote:
>  > > > >>> Dear Guoqing,
>  > > > >>>
>  > > > >>> On 08.02.21 15:53, Guoqing Jiang wrote:
>  > > > >>>>
>  > > > >>>> On 2/8/21 12:38, Donald Buczek wrote:
>  > > > >>>>>> 5. maybe don't hold reconfig_mutex when try to unregister
>  > > > >>>>>> sync_thread, like this.
>  > > > >>>>>>
>  > > > >>>>>>           /* resync has finished, collect result */
>  > > > >>>>>>           mddev_unlock(mddev);
>  > > > >>>>>>           md_unregister_thread(&mddev->sync_thread);
>  > > > >>>>>>           mddev_lock(mddev);
>  > > > >>>>> As above: While we wait for the sync thread to terminate, 
> would=
> n't it
>  > > > >>>>> be a problem, if another user space operation takes the mutex?
>  > > > >>>> I don't think other places can be blocked while hold mutex, 
> othe=
> rwise
>  > > > >>>> these places can cause potential deadlock. Please try above 
> two =
> lines
>  > > > >>>> change. And perhaps others have better idea.
>  > > > >>> Yes, this works. No deadlock after >11000 seconds,
>  > > > >>>
>  > > > >>> (Time till deadlock from previous runs/seconds: 1723, 37, 
> 434, 12=
> 65,
>  > > > >>> 3500, 1136, 109, 1892, 1060, 664, 84, 315, 12, 820 )
>  > > > >> Great. I will send a formal patch with your reported-by and 
> tested=
> -by.
>  > > > >>
>  > > > >> Thanks,
>  > > > >> Guoqing
>  > > > > I'm still hitting this issue with Linux 5.4.229 -- it looks 
> like 1/=
> 2
>  > > > > of the patches that supposedly resolve this were applied to the 
> sta=
> ble
>  > > > > kernels, however, one was omitted due to a regression:
>  > > > > md: don't unregister sync_thread with reconfig_mutex held 
> (upstream
>  > > > > commit 8b48ec23cc51a4e7c8dbaef5f34ebe67e1a80934)
>  > > > >
>  > > > > I don't see any follow-up on the thread from June 8th 2022 
> asking f=
> or
>  > > > > this patch to be dropped from all stable kernels since it caused a
>  > > > > regression.
>  > > > >
>  > > > > The patch doesn't appear to be present in the current mainline 
> kern=
> el
>  > > > > (6.3-rc2) either. So I assume this issue is still present 
> there, or=
>   it
>  > > > > was resolved differently and I just can't find the commit/patch.
>  > > >
>  > > > It should be fixed by commit 9dfbdafda3b3"md: unlock mddev before 
> rea=
> p
>  > > > sync_thread in action_store".
>  > >
>  > > Okay, let me try applying that patch... it does not appear to be
>  > > present in my 5.4.229 kernel source. Thanks.
>  >
>  > Yes, applying this '9dfbdafda3b3 "md: unlock mddev before reap
>  > sync_thread in action_store"' patch on top of vanilla 5.4.229 source
>  > appears to fix the problem for me -- I can't reproduce the issue with
>  > the script, and it's been running for >24 hours now. (Previously I was
>  > able to induce the issue within a matter of minutes.)
> 
> Hi Marc,
> 
> Could you please run your reproducer on the md-tmp branch?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/log/?h=3Dmd-tmp
> 
> This contains a different version of the fix by Yu Kuai.
> 
> Thanks,
> Song
> 

Hi Song, I can easily reproduce this issue on 5.10.133 and 5.10.53. The change
"9dfbdafda3b3 "md: unlock mddev before reap" does not fix the issue for me.

But I did pull the changes from the md-tmp branch you are refering:
https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/log/?h=3Dmd-tmp

I was not totally clear on which change exactly to pull, but I pulled
the following changes:
2023-03-28 md: enhance checking in md_check_recovery()md-tmp	Yu Kuai	1 -7/+15
2023-03-28 md: wake up 'resync_wait' at last in md_reap_sync_thread()	Yu Kuai	1 -1/+1
2023-03-28 md: refactor idle/frozen_sync_thread()	Yu Kuai	2 -4/+22
2023-03-28 md: add a mutex to synchronize idle and frozen in action_store()	Yu Kuai	2 -0/+8
2023-03-28 md: refactor action_store() for 'idle' and 'frozen'	Yu Kuai	1 -16/+45

I used to be able to reproduce the lockup within minutes, but with those
changes the test system has been running for more than 120 hours.

When you said a "different fix", can you confirm that I grabbed the right
changes and that I need all 5 of them.

And second question was, has this fix been submitted upstream yet?
If so which kernel version?

Thank you