block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old in
atomic_inc_below. 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>
Link: https://lore.kernel.org/r/20220712150547.5786-1-ubizjak@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index d3a75693adbf4ddf9ffb94b07d4302ffcd2ea0cb..88f0fe7dcf5451915cb950b3dc4cbd88ad5b7696 100644 (file)
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -10,16 +10,10 @@ static bool atomic_inc_below(atomic_t *v, unsigned int below)
 {
        unsigned int cur = atomic_read(v);
 
-       for (;;) {
-               unsigned int old;
-
+       do {
                if (cur >= below)
                        return false;
-               old = atomic_cmpxchg(v, cur, cur + 1);
-               if (old == cur)
-                       break;
-               cur = old;
-       }
+       } while (!atomic_try_cmpxchg(v, &cur, cur + 1));
 
        return true;
 }