Re: [PATCH] md: optimize barrier usage for Rmw atomic bitops

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 <redacted>
---
  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.
+		 */

+		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
-			   */