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

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Message-ID: <CAPhsuW7kkSMNpo9cm3L23o7T676iLa=7gq8V9YkCx0MA6ad+QQ@mail.gmail.com>
Date:   Fri, 24 Nov 2023 09:36:35 -0800
From:   Song Liu <song@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     xni@...hat.com, 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] md: synchronize flush io with array reconfiguration

On Wed, Nov 8, 2023 at 2:07 AM 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>

LGTM.

> ---
>  drivers/md/md.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 4ee4593c874a..eb3e455bcbae 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -529,6 +529,9 @@ 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);
>         }
> @@ -548,12 +551,8 @@ 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,
> @@ -564,7 +563,6 @@ 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))
> @@ -617,6 +615,17 @@ 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.
> +                */
> +               percpu_ref_tryget(&mddev->active_io);

Probably add an warn_on here to catch any issues in the future.

Thanks,
Song

>                 mddev->flush_bio = bio;
>                 bio = NULL;
>         }
> --
> 2.39.2
>