linux-kernel - [PATCH v2] block: Use try_cmpxchg in update_io

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Date:   Tue, 12 Jul 2022 17:27:41 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Jens Axboe <axboe@...nel.dk>
Subject: [PATCH v2] block: Use try_cmpxchg in update_io_ticks

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
update_io_ticks. x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related
move instruction in front of cmpxchg).

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>
---
v2: Split patch from the original big patch
---
 block/blk-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 27fb1357ad4b..628b965356db 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -987,7 +987,7 @@ void update_io_ticks(struct block_device *part, unsigned long now, bool end)
 again:
 	stamp = READ_ONCE(part->bd_stamp);
 	if (unlikely(time_after(now, stamp))) {
-		if (likely(cmpxchg(&part->bd_stamp, stamp, now) == stamp))
+		if (likely(try_cmpxchg(&part->bd_stamp, &stamp, now)))
 			__part_stat_add(part, io_ticks, end ? now - stamp : 1);
 	}
 	if (part->bd_partno) {
-- 
2.35.3