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

[Yu Kuai]

在 2023/03/14 21:55, Guoqing Jiang 写道:
> On 3/14/23 21:25, Marc Smith wrote:
> > 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)
Hi, Guoqing,

Just borrow this thread to discuss, I think this commit might have
problem in some corner cases:

t1:				t2:
action_store
 mddev_lock
  if (mddev->sync_thread)
   mddev_unlock
   md_unregister_thread
				md_check_recovery
				 set_bit(MD_RECOVERY_RUNNING, &mddev->recovery)
				 queue_work(md_misc_wq, &mddev->del_work)
   mddev_lock_nointr
   md_reap_sync_thread
   // clear running
 mddev_lock

t3:
md_start_sync
// running is not set

Our test report a problem that can be cause by this in theory, by we
can't be sure for now...

We thought about how to fix this, instead of calling
md_register_thread() here to wait for sync_thread to be done
synchronisely, we do this asynchronously like what md_set_readonly() and
do_md_stop() does.

What do you think?

Thanks,
Kuai
> > 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.
>
> It should be fixed by commit 9dfbdafda3b3"md: unlock mddev before reap
> sync_thread in action_store".
>
> Thanks,
> Guoqing
> .