linux-kernel - [PATCH] zram: use try_cmpxchg in update_used

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Message-Id: <20221018145154.3699-1-ubizjak@gmail.com>
Date:   Tue, 18 Oct 2022 16:51:54 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] zram: use try_cmpxchg in update_used_max

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
update_used_max.  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, reorder code a bit to remove additional compare and
conditional jump from the assembly code.  Together, hese two
changes save 15 bytes from the function when compiled for x86_64.

No functional change intended.

Cc: Minchan Kim <minchan@...nel.org>
Cc: Nitin Gupta <ngupta@...are.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 drivers/block/zram/zram_drv.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 966aab902d19..87711ddf4b54 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -188,16 +188,13 @@ static void update_position(u32 *index, int *offset, struct bio_vec *bvec)
 static inline void update_used_max(struct zram *zram,
 					const unsigned long pages)
 {
-	unsigned long old_max, cur_max;
-
-	old_max = atomic_long_read(&zram->stats.max_used_pages);
+	unsigned long cur_max = atomic_long_read(&zram->stats.max_used_pages);
 
 	do {
-		cur_max = old_max;
-		if (pages > cur_max)
-			old_max = atomic_long_cmpxchg(
-				&zram->stats.max_used_pages, cur_max, pages);
-	} while (old_max != cur_max);
+		if (cur_max >= pages)
+			return;
+	} while (!atomic_long_try_cmpxchg(&zram->stats.max_used_pages,
+					  &cur_max, pages));
 }
 
 static inline void zram_fill_page(void *ptr, unsigned long len,
-- 
2.37.3