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.

Link: https://lkml.kernel.org/r/20221018145154.3699-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 966aab9..87711dd 100644 (file)
--- 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,