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SubjectRe: [PATCH] md: optimize barrier usage for Rmw atomic bitops
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On 1/29/20 7:14 PM, Davidlohr Bueso wrote:
> For both set and clear_bit, we can avoid the unnecessary barrier
> on non LL/SC architectures, such as x86. Instead, use the
> smp_mb__{before,after}_atomic() calls.
>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
> drivers/md/md.c | 2 +-
> drivers/md/raid10.c | 7 ++++---
> drivers/md/raid5.c | 9 +++++----
> 3 files changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 4824d50526fa..4ed2eb6933f7 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -2561,7 +2561,7 @@ static bool set_in_sync(struct mddev *mddev)
> * Ensure ->in_sync is visible before we clear
> * ->sync_checkers.
> */
> - smp_mb();
> + smp_mb__before_atomic();
> set_bit(MD_SB_CHANGE_CLEAN, &mddev->sb_flags);
> sysfs_notify_dirent_safe(mddev->sysfs_state);
> }
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index ec136e44aef7..1993a1958c75 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1865,9 +1865,10 @@ static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> /* We must have just cleared 'rdev' */
> p->rdev = p->replacement;
> clear_bit(Replacement, &p->replacement->flags);
> - smp_mb(); /* Make sure other CPUs may see both as identical
> - * but will never see neither -- if they are careful.
> - */
> + /* Make sure other CPUs may see both as identical
> + * but will never see neither -- if they are careful.
> + */

Since we are here, it is better to change the comment style to

/*
* ...
*/

> + smp_mb__after_atomic();
> p->replacement = NULL;
> }
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index ba00e9877f02..3ad6209287cf 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -364,7 +364,7 @@ static int release_stripe_list(struct r5conf *conf,
> int hash;
>
> /* sh could be readded after STRIPE_ON_RELEASE_LIST is cleard */
> - smp_mb();
> + smp_mb__before_atomic();
> clear_bit(STRIPE_ON_RELEASE_LIST, &sh->state);
> /*
> * Don't worry the bit is set here, because if the bit is set
> @@ -7654,9 +7654,10 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
> /* We must have just cleared 'rdev' */
> p->rdev = p->replacement;
> clear_bit(Replacement, &p->replacement->flags);
> - smp_mb(); /* Make sure other CPUs may see both as identical
> - * but will never see neither - if they are careful
> - */

Ditto.

Acked-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>

Thanks,
Guoqing

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