2022-07-12 15:24:42

by Uros Bizjak

Subject: [PATCH v2] 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 <[email protected]>
Cc: Jens Axboe <[email protected]>
---
v2: Split patch from the original big patch
---
block/blk-rq-qos.c | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index d3a75693adbf..88f0fe7dcf54 100644
--- 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;
}
--
2.35.3

2022-07-12 20:42:30

by Jens Axboe

Subject: Re: [PATCH v2] block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below

On Tue, 12 Jul 2022 17:05:47 +0200, Uros Bizjak wrote:
> 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.
>
> [...]

Applied, thanks!

[1/1] block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below
commit: f4b1e27db49c8b985b116aa99481b4c6a4342ed4

Best regards,
--
Jens Axboe