* [PATCH v2] block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below
@ 2022-07-12 15:05 Uros Bizjak
2022-07-12 20:39 ` Jens Axboe
0 siblings, 1 reply; 2+ messages in thread
From: Uros Bizjak @ 2022-07-12 15:05 UTC (permalink / raw)
To: linux-block, linux-kernel; +Cc: Uros Bizjak, Jens Axboe
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>
---
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v2] block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below
2022-07-12 15:05 [PATCH v2] block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below Uros Bizjak
@ 2022-07-12 20:39 ` Jens Axboe
0 siblings, 0 replies; 2+ messages in thread
From: Jens Axboe @ 2022-07-12 20:39 UTC (permalink / raw)
To: ubizjak, linux-kernel, linux-block
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
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2022-07-12 15:05 [PATCH v2] block/rq_qos: Use atomic_try_cmpxchg in atomic_inc_below Uros Bizjak
2022-07-12 20:39 ` Jens Axboe
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