linux-kernel - Re: [PATCH -next v2] md: synchronize flush io with array reconfiguration

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Message-ID: <CAPhsuW57SuytxCY-fV74qx6B8AYb65nFC_t2VVeTN34Pamp=gQ@mail.gmail.com>
Date:   Mon, 27 Nov 2023 15:32:59 -0800
From:   Song Liu <song@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     maan@...temlinux.org, neilb@...e.de, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next v2] md: synchronize flush io with array reconfiguration

On Mon, Nov 27, 2023 at 2:16 PM Song Liu <song@...nel.org> wrote:
>
> On Fri, Nov 24, 2023 at 10:54 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
> >
> > From: Yu Kuai <yukuai3@...wei.com>
> >
> > Currently rcu is used to protect iterating rdev from submit_flushes():
> >
> > submit_flushes                  remove_and_add_spares
> >                                 synchronize_rcu
> >                                 pers->hot_remove_disk()
> >  rcu_read_lock()
> >  rdev_for_each_rcu
> >   if (rdev->raid_disk >= 0)
> >                                 rdev->radi_disk = -1;
> >    atomic_inc(&rdev->nr_pending)
> >    rcu_read_unlock()
> >    bi = bio_alloc_bioset()
> >    bi->bi_end_io = md_end_flush
> >    bi->private = rdev
> >    submit_bio
> >    // issue io for removed rdev
> >
> > Fix this problem by grabbing 'acive_io' before iterating rdev, make sure
> > that remove_and_add_spares() won't concurrent with submit_flushes().
> >
> > Fixes: a2826aa92e2e ("md: support barrier requests on all personalities.")
> > Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> > ---
> > Changes v2:
> >  - Add WARN_ON in case md_flush_request() is not called from
> >  md_handle_request() in future.
> >
> >  drivers/md/md.c | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/md/md.c b/drivers/md/md.c
> > index 86efc9c2ae56..2ffedc39edd6 100644
> > --- a/drivers/md/md.c
> > +++ b/drivers/md/md.c
> > @@ -538,6 +538,9 @@ static void md_end_flush(struct bio *bio)
> >         rdev_dec_pending(rdev, mddev);
> >
> >         if (atomic_dec_and_test(&mddev->flush_pending)) {
> > +               /* The pair is percpu_ref_tryget() from md_flush_request() */
> > +               percpu_ref_put(&mddev->active_io);
> > +
> >                 /* The pre-request flush has finished */
> >                 queue_work(md_wq, &mddev->flush_work);
> >         }
> > @@ -557,12 +560,8 @@ static void submit_flushes(struct work_struct *ws)
> >         rdev_for_each_rcu(rdev, mddev)
> >                 if (rdev->raid_disk >= 0 &&
> >                     !test_bit(Faulty, &rdev->flags)) {
> > -                       /* Take two references, one is dropped
> > -                        * when request finishes, one after
> > -                        * we reclaim rcu_read_lock
> > -                        */
> >                         struct bio *bi;
> > -                       atomic_inc(&rdev->nr_pending);
> > +
> >                         atomic_inc(&rdev->nr_pending);
> >                         rcu_read_unlock();
> >                         bi = bio_alloc_bioset(rdev->bdev, 0,
> > @@ -573,7 +572,6 @@ static void submit_flushes(struct work_struct *ws)
> >                         atomic_inc(&mddev->flush_pending);
> >                         submit_bio(bi);
> >                         rcu_read_lock();
> > -                       rdev_dec_pending(rdev, mddev);
> >                 }
> >         rcu_read_unlock();
> >         if (atomic_dec_and_test(&mddev->flush_pending))
> > @@ -626,6 +624,18 @@ bool md_flush_request(struct mddev *mddev, struct bio *bio)
> >         /* new request after previous flush is completed */
> >         if (ktime_after(req_start, mddev->prev_flush_start)) {
> >                 WARN_ON(mddev->flush_bio);
> > +               /*
> > +                * Grab a reference to make sure mddev_suspend() will wait for
> > +                * this flush to be done.
> > +                *
> > +                * md_flush_reqeust() is called under md_handle_request() and
> > +                * 'active_io' is already grabbed, hence percpu_ref_tryget()
> > +                * won't fail, percpu_ref_tryget_live() can't be used because
> > +                * percpu_ref_kill() can be called by mddev_suspend()
> > +                * concurrently.
> > +                */
> > +               if (WARN_ON(percpu_ref_tryget(&mddev->active_io)))
>
> This should be "if (!WARN_ON(..))", right?
>
> Song
>
> > +                       percpu_ref_get(&mddev->active_io);

Actually, we can just use percpu_ref_get(), no?

Thanks,
Song