Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle" transition

[Marc Smith]

On Mon, Feb 8, 2021 at 7:49 PM Guoqing Jiang
<guoqing.jiang@xxxxxxxxxxxxxxx> 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, wouldn'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, otherwise
> >> 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, 1265,
> > 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 stable
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 for
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 kernel
(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.

I can induce the issue by using Donald's script above which will
eventually result in hangs:
...
147948.504621] INFO: task md_test_2.sh:68033 blocked for more than 122 seconds.
[147948.504624]       Tainted: P           OE     5.4.229-esos.prod #1
[147948.504624] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[147948.504625] md_test_2.sh    D    0 68033      1 0x00000004
[147948.504627] Call Trace:
[147948.504634]  __schedule+0x4ab/0x4f3
[147948.504637]  ? usleep_range+0x7a/0x7a
[147948.504638]  schedule+0x67/0x81
[147948.504639]  schedule_timeout+0x2c/0xe5
[147948.504643]  ? do_raw_spin_lock+0x2b/0x52
[147948.504644]  __wait_for_common+0xc4/0x13a
[147948.504647]  ? wake_up_q+0x40/0x40
[147948.504649]  kthread_stop+0x9a/0x117
[147948.504653]  md_unregister_thread+0x43/0x4d
[147948.504655]  md_reap_sync_thread+0x1c/0x1d5
[147948.504657]  action_store+0xc9/0x284
[147948.504658]  md_attr_store+0x9f/0xb8
[147948.504661]  kernfs_fop_write+0x10a/0x14c
[147948.504664]  vfs_write+0xa0/0xdd
[147948.504666]  ksys_write+0x71/0xba
[147948.504668]  do_syscall_64+0x52/0x60
[147948.504671]  entry_SYSCALL_64_after_hwframe+0x5c/0xc1
...
[147948.504748] INFO: task md120_resync:135315 blocked for more than
122 seconds.
[147948.504749]       Tainted: P           OE     5.4.229-esos.prod #1
[147948.504749] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[147948.504749] md120_resync    D    0 135315      2 0x80004000
[147948.504750] Call Trace:
[147948.504752]  __schedule+0x4ab/0x4f3
[147948.504754]  ? printk+0x53/0x6a
[147948.504755]  schedule+0x67/0x81
[147948.504756]  md_do_sync+0xae7/0xdd9
[147948.504758]  ? remove_wait_queue+0x41/0x41
[147948.504759]  md_thread+0x128/0x151
[147948.504761]  ? _raw_spin_lock_irqsave+0x31/0x5d
[147948.504762]  ? md_start_sync+0xdc/0xdc
[147948.504763]  kthread+0xe4/0xe9
[147948.504764]  ? kthread_flush_worker+0x70/0x70
[147948.504765]  ret_from_fork+0x35/0x40
...

This happens on 'raid6' MD RAID arrays that initially have sync_action==resync.

Any guidance would be greatly appreciated.

--Marc