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

[Yu Kuai]

Hi,

在 2023/06/14 11:47, Xiao Ni 写道:
> 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?

r10_bio will always be added to bio_end_io_list, no matter
narrow_write_error() succeed or not. The dependecy chain 1 here is just
indicate handle this r10_bio will wait for updating super block, it's
not where MD_RECOVERY_PENDING is set...

And MD_RECOVERY_PENDING can be set from narrow_write_error() and other
places where rdev_set_badblocks() is called.
> >     // later from md thread
> >     raid10d
> >      if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
	-> It's here, if the flag is set, bio won't be handled.
> >       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.

It's not that bio can't be added to bio_end_io_list, it's that bio in
this list can't be handled if sb_flags is set.

Thanks,
Kuai
> 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
>
> .