Re: [PATCH 1/1] md/raid10: Remove rcu_dereference when it doesn't need rcu lock to protect

[Song Liu <song@kernel.org>]

On Sun, Aug 8, 2021 at 9:02 PM Xiao Ni <xni@redhat.com> wrote:
>
> 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. 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.
>
> Signed-off-by: Xiao Ni <xni@redhat.com>

Will we get performance benefits from this change? If not, I would
prefer to keep the code as-is.

Thanks,
Song

> ---
>  drivers/md/raid10.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 16977e8..cef9869 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1743,9 +1743,8 @@ 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);
> +               struct md_rdev *rdev = conf->mirrors[disk].rdev;
> +               struct md_rdev *rrdev = conf->mirrors[disk].replacement;
>
>                 /*
>                  * Now start to calculate the start and end address for each disk.
> --
> 2.7.5
>