Re: [PATCH v2] md/raid10: Remove rcu_dereference when it doesn't need rcu lock to protect

[Song Liu <song@kernel.org>]

On Tue, Aug 17, 2021 at 10:58 PM Xiao Ni <xni@redhat.com> wrote:
>
> One warning message is triggered like this:
> [  695.110751] =============================
> [  695.131439] WARNING: suspicious RCU usage
> [  695.151389] 4.18.0-319.el8.x86_64+debug #1 Not tainted
> [  695.174413] -----------------------------
> [  695.192603] drivers/md/raid10.c:1776 suspicious
> rcu_dereference_check() usage!
> [  695.225107] other info that might help us debug this:
> [  695.260940] rcu_scheduler_active = 2, debug_locks = 1
> [  695.290157] no locks held by mkfs.xfs/10186.
>
> In the first loop of function raid10_handle_discard. It already
> determines which disk need to handle discard request and add the
> rdev reference count rdev->nr_pending. So the conf->mirrors will
> not change until all bios come back from underlayer disks. It
> doesn't need to use rcu_dereference to get rdev.
>
> Fixes: d30588b2731f ('md/raid10: improve raid10 discard request')
> Signed-off-by: Xiao Ni <xni@redhat.com>
> Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>

Applied to md-fixes. Thanks!
Song

> ---
> V2: Fix comment style problem
>
>  drivers/md/raid10.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 16977e8..d5d9233 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1712,6 +1712,11 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
>         } else
>                 r10_bio->master_bio = (struct bio *)first_r10bio;
>
> +       /*
> +        * first select target devices under rcu_lock and
> +        * inc refcount on their rdev.  Record them by setting
> +        * bios[x] to bio
> +        */
>         rcu_read_lock();
>         for (disk = 0; disk < geo->raid_disks; disk++) {
>                 struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev);
> @@ -1743,9 +1748,6 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
>         for (disk = 0; disk < geo->raid_disks; disk++) {
>                 sector_t dev_start, dev_end;
>                 struct bio *mbio, *rbio = NULL;
> -               struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev);
> -               struct md_rdev *rrdev = rcu_dereference(
> -                       conf->mirrors[disk].replacement);
>
>                 /*
>                  * Now start to calculate the start and end address for each disk.
> @@ -1775,9 +1777,12 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
>
>                 /*
>                  * It only handles discard bio which size is >= stripe size, so
> -                * dev_end > dev_start all the time
> +                * dev_end > dev_start all the time.
> +                * It doesn't need to use rcu lock to get rdev here. We already
> +                * add rdev->nr_pending in the first loop.
>                  */
>                 if (r10_bio->devs[disk].bio) {
> +                       struct md_rdev *rdev = conf->mirrors[disk].rdev;
>                         mbio = bio_clone_fast(bio, GFP_NOIO, &mddev->bio_set);
>                         mbio->bi_end_io = raid10_end_discard_request;
>                         mbio->bi_private = r10_bio;
> @@ -1790,6 +1795,7 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
>                         bio_endio(mbio);
>                 }
>                 if (r10_bio->devs[disk].repl_bio) {
> +                       struct md_rdev *rrdev = conf->mirrors[disk].replacement;
>                         rbio = bio_clone_fast(bio, GFP_NOIO, &mddev->bio_set);
>                         rbio->bi_end_io = raid10_end_discard_request;
>                         rbio->bi_private = r10_bio;
> --
> 2.7.5
>