Re: [dm-devel] [PATCH -next v2 4/6] md: refactor idle/frozen_sync_thread() to fix deadlock

[Xiao Ni]

On Wed, Jun 14, 2023 at 9:48 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2023/06/13 22:50, Xiao Ni 写道:
> >
> > 在 2023/5/29 下午9:20, Yu Kuai 写道:
> >> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >>
> >> Our test found a following deadlock in raid10:
> >>
> >> 1) Issue a normal write, and such write failed:
> >>
> >>    raid10_end_write_request
> >>     set_bit(R10BIO_WriteError, &r10_bio->state)
> >>     one_write_done
> >>      reschedule_retry
> >>
> >>    // later from md thread
> >>    raid10d
> >>     handle_write_completed
> >>      list_add(&r10_bio->retry_list, &conf->bio_end_io_list)
> >>
> >>    // later from md thread
> >>    raid10d
> >>     if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
> >>      list_move(conf->bio_end_io_list.prev, &tmp)
> >>      r10_bio = list_first_entry(&tmp, struct r10bio, retry_list)
> >>      raid_end_bio_io(r10_bio)
> >>
> >> Dependency chain 1: normal io is waiting for updating superblock
> >
> > Hi Kuai
> >
> > It looks like the above situation is more complex. It only needs a
> > normal write and md_write_start needs to
> >
> > wait until the metadata is written to member disks, right? If so, it
> > doesn't need to introduce raid10 write failure
> >
> > here. I guess, it should be your test case. It's nice, if you can put
> > your test steps in the patch. But for the analysis
> >
> > of the deadlock here, it's better to be simple.
>
> Test script can be found here, it's pretty easy to trigger:
>
> https://patchwork.kernel.org/project/linux-raid/patch/20230529132826.2125392-4-yukuai1@xxxxxxxxxxxxxxx/

Thanks for this.
>
> While reviewing the related code, I found that io can only be added to
> list bio_end_io_list from handle_write_completed() if such io failed, so
> I think io failure is needed to trigger deadlock from daemon thread.
>
> I think the key point is how MD_SB_CHANGE_PENDING is set:
>
> 1) raid10_error() and rdev_set_badblocks(), trigger by io failure;
> 2) raid10_write_request() related to reshape;
> 3) md_write_start() and md_allow_write(), and mddev->in_sync is set,
> however, I was thinking this is not a common case;
>
> 1) is used here because it's quite easy to trigger and this is what
> we meet in real test. 3) is possible but I will say let's keep 1), I
> don't think it's necessary to reporduce this deadlock through another
> path again.

It makes sense. Let's go back to the first path mentioned in the patch.

> 1) Issue a normal write, and such write failed:
>
>    raid10_end_write_request
>     set_bit(R10BIO_WriteError, &r10_bio->state)
>     one_write_done
>      reschedule_retry

This is good.
>
>    // later from md thread
>    raid10d
>     handle_write_completed
>      list_add(&r10_bio->retry_list, &conf->bio_end_io_list)

I have a question here. It should run narrow_write_error in
handle_write_completed. In the test case, will narrow_write_error run
successfully? Or it fails and will call rdev_set_badblocks and
md_error. So MD_RECOVERY_PENDING will be set?

>
>    // later from md thread
>    raid10d
>     if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
>      list_move(conf->bio_end_io_list.prev, &tmp)
>      r10_bio = list_first_entry(&tmp, struct r10bio, retry_list)
>      raid_end_bio_io(r10_bio)
>
> Dependency chain 1: normal io is waiting for updating superblock

It's a little hard to understand. Because it doesn't show how normal
io waits for a superblock update. And based on your last email, I
guess you want to say rdev_set_badblock sets MD_RECOVERY_PENDING, but
the flag can't be cleared, so the bios can't be added to
bio_end_io_list, so the io rquests can't be finished.

Regards
Xiao
>
> Thanks,
> Kuai
> >
> >>
> >> 2) Trigger a recovery:
> >>
> >>    raid10_sync_request
> >>     raise_barrier
> >>
> >> Dependency chain 2: sync thread is waiting for normal io
> >>
> >> 3) echo idle/frozen to sync_action:
> >>
> >>    action_store
> >>     mddev_lock
> >>      md_unregister_thread
> >>       kthread_stop
> >>
> >> Dependency chain 3: drop 'reconfig_mutex' is waiting for sync thread
> >>
> >> 4) md thread can't update superblock:
> >>
> >>    raid10d
> >>     md_check_recovery
> >>      if (mddev_trylock(mddev))
> >>       md_update_sb
> >>
> >> Dependency chain 4: update superblock is waiting for 'reconfig_mutex'
> >>
> >> Hence cyclic dependency exist, in order to fix the problem, we must
> >> break one of them. Dependency 1 and 2 can't be broken because they are
> >> foundation design. Dependency 4 may be possible if it can be guaranteed
> >> that no io can be inflight, however, this requires a new mechanism which
> >> seems complex. Dependency 3 is a good choice, because idle/frozen only
> >> requires sync thread to finish, which can be done asynchronously that is
> >> already implemented, and 'reconfig_mutex' is not needed anymore.
> >>
> >> This patch switch 'idle' and 'frozen' to wait sync thread to be done
> >> asynchronously, and this patch also add a sequence counter to record how
> >> many times sync thread is done, so that 'idle' won't keep waiting on new
> >> started sync thread.
> >
> > 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?
> >
> > Regards
> >
> > Xiao
> >
> >
> >>
> >> Noted that raid456 has similiar deadlock([1]), and it's verified[2] this
> >> deadlock can be fixed by this patch as well.
> >>
> >> [1]
> >> https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@xxxxxxxxxxxxx/T/#t
> >>
> >> [2]
> >> https://lore.kernel.org/linux-raid/e9067438-d713-f5f3-0d3d-9e6b0e9efa0e@xxxxxxxxxxxxxxx/
> >>
> >> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> >> ---
> >>   drivers/md/md.c | 23 +++++++++++++++++++----
> >>   drivers/md/md.h |  2 ++
> >>   2 files changed, 21 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/md/md.c b/drivers/md/md.c
> >> index 63a993b52cd7..7912de0e4d12 100644
> >> --- a/drivers/md/md.c
> >> +++ b/drivers/md/md.c
> >> @@ -652,6 +652,7 @@ void mddev_init(struct mddev *mddev)
> >>       timer_setup(&mddev->safemode_timer, md_safemode_timeout, 0);
> >>       atomic_set(&mddev->active, 1);
> >>       atomic_set(&mddev->openers, 0);
> >> +    atomic_set(&mddev->sync_seq, 0);
> >>       spin_lock_init(&mddev->lock);
> >>       atomic_set(&mddev->flush_pending, 0);
> >>       init_waitqueue_head(&mddev->sb_wait);
> >> @@ -4776,19 +4777,27 @@ static void stop_sync_thread(struct mddev *mddev)
> >>       if (work_pending(&mddev->del_work))
> >>           flush_workqueue(md_misc_wq);
> >> -    if (mddev->sync_thread) {
> >> -        set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> >> -        md_reap_sync_thread(mddev);
> >> -    }
> >> +    set_bit(MD_RECOVERY_INTR, &mddev->recovery);
> >> +    /*
> >> +     * Thread might be blocked waiting for metadata update which will
> >> now
> >> +     * never happen
> >> +     */
> >> +    md_wakeup_thread_directly(mddev->sync_thread);
> >>       mddev_unlock(mddev);
> >>   }
> >>   static void idle_sync_thread(struct mddev *mddev)
> >>   {
> >> +    int sync_seq = atomic_read(&mddev->sync_seq);
> >> +
> >>       mutex_lock(&mddev->sync_mutex);
> >>       clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> >>       stop_sync_thread(mddev);
> >> +
> >> +    wait_event(resync_wait, sync_seq != atomic_read(&mddev->sync_seq) ||
> >> +            !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
> >> +
> >>       mutex_unlock(&mddev->sync_mutex);
> >>   }
> >> @@ -4797,6 +4806,10 @@ static void frozen_sync_thread(struct mddev
> >> *mddev)
> >>       mutex_init(&mddev->delete_mutex);
> >>       set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> >>       stop_sync_thread(mddev);
> >> +
> >> +    wait_event(resync_wait, mddev->sync_thread == NULL &&
> >> +            !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
> >> +
> >>       mutex_unlock(&mddev->sync_mutex);
> >>   }
> >> @@ -9472,6 +9485,8 @@ void md_reap_sync_thread(struct mddev *mddev)
> >>       /* resync has finished, collect result */
> >>       md_unregister_thread(&mddev->sync_thread);
> >> +    atomic_inc(&mddev->sync_seq);
> >> +
> >>       if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
> >>           !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
> >>           mddev->degraded != mddev->raid_disks) {
> >> diff --git a/drivers/md/md.h b/drivers/md/md.h
> >> index 2fa903de5bd0..7cab9c7c45b8 100644
> >> --- a/drivers/md/md.h
> >> +++ b/drivers/md/md.h
> >> @@ -539,6 +539,8 @@ struct mddev {
> >>       /* Used to synchronize idle and frozen for action_store() */
> >>       struct mutex            sync_mutex;
> >> +    /* The sequence number for sync thread */
> >> +    atomic_t sync_seq;
> >>       bool    has_superblocks:1;
> >>       bool    fail_last_dev:1;
> >
> > --
> > dm-devel mailing list
> > dm-devel@xxxxxxxxxx
> > https://listman.redhat.com/mailman/listinfo/dm-devel
>