Re: [PATCH -next 1/6] Revert "md: unlock mddev before reap sync_thread in action_store"

[Yu Kuai]

Hi,

在 2023/03/22 22:32, Guoqing Jiang 写道:
> > > Could you explain how the same work can be re-queued? Isn't the 
> > > PENDING_BIT
> > > is already set in t3? I believe queue_work shouldn't do that per the 
> > > comment
> > > but I am not expert ...
> >
> > This is not related to workqueue, it is just because raid10
> > reinitialize the work that is already queued, 
>
> I am trying to understand the possibility.
>
> > like I discribed later in t3:
> >
> > t2:
> > md_check_recovery:
> >  INIT_WORK -> clear pending
> >  queue_work -> set pending
> >   list_add_tail
> > ...
> >
> > t3: -> work is still pending
> > md_check_recovery:
> >  INIT_WORK -> clear pending
> >  queue_work -> set pending
> >   list_add_tail -> list is corrupted
>
> First, t2 and t3 can't be run in parallel since reconfig_mutex must be 
> held. And if sync_thread existed,
> the second process would unregister and reap sync_thread which means the 
> second process will
> call INIT_WORK and queue_work again.
>
> Maybe your description is valid, I would prefer call work_pending and 
> flush_workqueue instead of
> INIT_WORK and queue_work.

This is not enough, it's right this can avoid list corruption, but the
worker function md_start_sync just register a sync_thread, and
md_do_sync() can still in progress, hence this can't prevent a new
sync_thread to start while the old one is not done, some other problems
like deadlock can still be triggered.

> > Of course, our 5.10 and mainline are the same,
> >
> > there are some tests:
> >
> > First the deadlock can be reporduced reliably, test script is simple:
> >
> > mdadm -Cv /dev/md0 -n 4 -l10 /dev/sd[abcd]
>
> So this is raid10 while the previous problem was appeared in raid456, I 
> am not sure it is the same
> issue, but let's see.

Ok, I'm not quite familiar with raid456 yet, however, the problem is
still related to that action_store hold mutex to unregister sync_thread,
right?

> > Then, the problem MD_RECOVERY_RUNNING can be cleared can't be reporduced
> > reliably, usually it takes 2+ days to triggered a problem, and each time
> > problem phenomenon can be different, I'm hacking the kernel and add
> > some BUG_ON to test MD_RECOVERY_RUNNING in attached patch, following
> > test can trigger the BUG_ON:
>
> Also your debug patch obviously added large delay which make the 
> calltrace happen, I doubt
> if user can hit it in real life. Anyway, will try below test from my side.
>
> > mdadm -Cv /dev/md0 -e1.0 -n 4 -l 10 /dev/sd{a..d} --run
> > sleep 5
> > echo 1 > /sys/module/md_mod/parameters/set_delay
> > echo idle > /sys/block/md0/md/sync_action &
> > sleep 5
> > echo "want_replacement" > /sys/block/md0/md/dev-sdd/state
> >
> > test result:
> >
> > [  228.390237] md_check_recovery: running is set
> > [  228.391376] md_check_recovery: queue new sync thread
> > [  233.671041] action_store unregister success! delay 10s
> > [  233.689276] md_check_recovery: running is set
> > [  238.722448] md_check_recovery: running is set
> > [  238.723328] md_check_recovery: queue new sync thread
> > [  238.724851] md_do_sync: before new wor, sleep 10s
> > [  239.725818] md_do_sync: delay done
> > [  243.674828] action_store delay done
> > [  243.700102] md_reap_sync_thread: running is cleared!
> > [  243.748703] ------------[ cut here ]------------
> > [  243.749656] kernel BUG at drivers/md/md.c:9084!
>
> After your debug patch applied, is L9084 points to below?
>
> 9084                                 mddev->curr_resync = MaxSector;

In my environment, it's a BUG_ON() that I added in md_do_sync:

9080  skip:
9081         /* set CHANGE_PENDING here since maybe another update is 
needed,
9082         ┊* so other nodes are informed. It should be harmless for 
normal
9083         ┊* raid */
9084         BUG_ON(!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
9085         set_mask_bits(&mddev->sb_flags, 0,
9086                 ┊     BIT(MD_SB_CHANGE_PENDING) | 
BIT(MD_SB_CHANGE_DEVS));

> I don't understand how it triggers below calltrace, and it has nothing 
> to do with
> list corruption, right?

Yes, this is just a early BUG_ON() to detect that if MD_RECOVERY_RUNNING
is cleared while sync_thread is still in progress.

Thanks,
Kuai