[PATCH v2] blk-cgroup: Use atomic{,64}_try_cmpxchg

[Uros Bizjak <ubizjak@gmail.com>]

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in blkcg_unuse_delay, blkcg_set_delay and blkcg_clear_delay and
atomic64_try_cmpxchg in blkcg_scale_delay.  x86 CMPXCHG instruction
returns success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg).

Also, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
---
v2: Split patch from the original big patch
    Use while instead of do-while loop
---
 block/blk-cgroup.c |  2 +-
 block/blk-cgroup.h | 12 ++++--------
 2 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 764e740b0c0f..ca99260348ff 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1696,7 +1696,7 @@ static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now)
 	 * everybody is happy with their IO latencies.
 	 */
 	if (time_before64(old + NSEC_PER_SEC, now) &&
-	    atomic64_cmpxchg(&blkg->delay_start, old, now) == old) {
+	    atomic64_try_cmpxchg(&blkg->delay_start, &old, now)) {
 		u64 cur = atomic64_read(&blkg->delay_nsec);
 		u64 sub = min_t(u64, blkg->last_delay, now - old);
 		int cur_use = atomic_read(&blkg->use_delay);
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index d4de0a35e066..d2724d1dd7c9 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -430,12 +430,8 @@ static inline int blkcg_unuse_delay(struct blkcg_gq *blkg)
 	 * then check to see if we were the last delay so we can drop the
 	 * congestion count on the cgroup.
 	 */
-	while (old) {
-		int cur = atomic_cmpxchg(&blkg->use_delay, old, old - 1);
-		if (cur == old)
-			break;
-		old = cur;
-	}
+	while (old && !atomic_try_cmpxchg(&blkg->use_delay, &old, old - 1))
+		;
 
 	if (old == 0)
 		return 0;
@@ -458,7 +454,7 @@ static inline void blkcg_set_delay(struct blkcg_gq *blkg, u64 delay)
 	int old = atomic_read(&blkg->use_delay);
 
 	/* We only want 1 person setting the congestion count for this blkg. */
-	if (!old && atomic_cmpxchg(&blkg->use_delay, old, -1) == old)
+	if (!old && atomic_try_cmpxchg(&blkg->use_delay, &old, -1))
 		atomic_inc(&blkg->blkcg->css.cgroup->congestion_count);
 
 	atomic64_set(&blkg->delay_nsec, delay);
@@ -475,7 +471,7 @@ static inline void blkcg_clear_delay(struct blkcg_gq *blkg)
 	int old = atomic_read(&blkg->use_delay);
 
 	/* We only want 1 person clearing the congestion count for this blkg. */
-	if (old && atomic_cmpxchg(&blkg->use_delay, old, 0) == old)
+	if (old && atomic_try_cmpxchg(&blkg->use_delay, &old, 0))
 		atomic_dec(&blkg->blkcg->css.cgroup->congestion_count);
 }
 
-- 
2.35.3