Re: [PATCH -next v2 02/28] md: use 'mddev->suspended' for is_md_suspended()

[Yu Kuai]

Hi,

在 2023/09/20 16:46, Xiao Ni 写道:
> On Mon, Aug 28, 2023 at 10:04 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > 'pers->prepare_suspend' is introduced to prevent a deadlock for raid456,
> > this change prepares to clean this up in later patches while refactoring
> > mddev_suspend(). Specifically allow reshape to make progress while
> > waiting for 'active_io' to be 0.
>
> Hi Kuai
>
> > From my side, I can't understand the comments. The change has
> relationship with pers->prepare_suspend? And why this change can
> affect reshape? If this change indeed can affect these two things, can
> you explain more?

First of all, 'prepare_suspend' is used to fix a deadlock in raid456:

1) suspend is waiting for normal io to be done.

mddev_suspend
 mddev->suspended++ -> new sync_thread can't start
 percpu_ref_kill(active_io)
 wait_event(percpu_ref_is_zero(active_io))

2) normal io is waiting for reshape to make progress.
3) reshape is waiting for suspended array to be resumed.

md_check_recovery
 if (is_md_suspended(mddev))
  return

Then prepare_suspend will failed the io that is waiting for reshape to
make progress:

mddev_suspend
 mddev->suspended++
 percpu_ref_kill(active_io)
  -> new io will be stuck in md_handle_request
 pers->prepare_suspend() -> raid5_prepare_suspend
  -> wake_up(wait_for_overlap)
		// woke up
		raid5_make_request
		 make_stripe_request
		  !reshape_inprogress(mddev) && reshape_disabled(mddev)
		   // return io error for the io that is waiting for
		   // reshape to make progress

 wait_event(percpu_ref_is_zero(active_io))

With this patch and the new api to suspend array:

mddev_suspend
 percpu_ref_kill(active_io)
 wait_event(percpu_ref_is_zero(active_io))
 -> while waiting for normal io to be done, new sync_thread can still
    start, and reshape can still make progress.
 mddev->suspended++

> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   drivers/md/md.c | 2 +-
> >   drivers/md/md.h | 2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/md/md.c b/drivers/md/md.c
> > index 9d8dff9d923c..7fa311a14317 100644
> > --- a/drivers/md/md.c
> > +++ b/drivers/md/md.c
> > @@ -355,7 +355,7 @@ static DEFINE_SPINLOCK(all_mddevs_lock);
> >    */
> >   static bool is_suspended(struct mddev *mddev, struct bio *bio)
> >   {
> > -       if (is_md_suspended(mddev))
> > +       if (is_md_suspended(mddev) || percpu_ref_is_dying(&mddev->active_io))
>
> If we use mddev->suspended to judge if the raid is suspended, it
> should be enough? Because mddev->suspended must be true when active_io
> is dying.

In the new api, active_io is killed before increasing suspended, and the
difference is that the timing that array is suspended will be delayed
from the start of mddev_suspend() to when all dispatched io is done.

I think this is OK because this doesn't change behaviour when
mddev_suspend() returns.

Thanks,
Kuai
`
> Best Regards
> Xiao
> >                  return true;
> >          if (bio_data_dir(bio) != WRITE)
> >                  return false;
> > diff --git a/drivers/md/md.h b/drivers/md/md.h
> > index b628c292506e..fb3b123f16dd 100644
> > --- a/drivers/md/md.h
> > +++ b/drivers/md/md.h
> > @@ -584,7 +584,7 @@ static inline bool md_is_rdwr(struct mddev *mddev)
> >
> >   static inline bool is_md_suspended(struct mddev *mddev)
> >   {
> > -       return percpu_ref_is_dying(&mddev->active_io);
> > +       return READ_ONCE(mddev->suspended);
> >   }
> >
> >   static inline int __must_check mddev_lock(struct mddev *mddev)
> > --
> > 2.39.2
>
> .