linux-kernel - [PATCH] dmaengine:idxd: Use local64_try_cmpxchg in perfmon_pmu_event

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Date:   Mon,  3 Jul 2023 16:52:37 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Fenghua Yu <fenghua.yu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>
Subject: [PATCH] dmaengine:idxd: Use local64_try_cmpxchg in perfmon_pmu_event_update

Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
in perfmon_pmu_event_update.  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, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc: Fenghua Yu <fenghua.yu@...el.com>
Cc: Dave Jiang <dave.jiang@...el.com>
Cc: Vinod Koul <vkoul@...nel.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 drivers/dma/idxd/perfmon.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/dma/idxd/perfmon.c b/drivers/dma/idxd/perfmon.c
index d73004f47cf4..fdda6d604262 100644
--- a/drivers/dma/idxd/perfmon.c
+++ b/drivers/dma/idxd/perfmon.c
@@ -245,12 +245,11 @@ static void perfmon_pmu_event_update(struct perf_event *event)
 	int shift = 64 - idxd->idxd_pmu->counter_width;
 	struct hw_perf_event *hwc = &event->hw;
 
+	prev_raw_count = local64_read(&hwc->prev_count);
 	do {
-		prev_raw_count = local64_read(&hwc->prev_count);
 		new_raw_count = perfmon_pmu_read_counter(event);
-	} while (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
-			new_raw_count) != prev_raw_count);
-
+	} while (!local64_try_cmpxchg(&hwc->prev_count,
+				      &prev_raw_count, new_raw_count));
 	n = (new_raw_count << shift);
 	p = (prev_raw_count << shift);
 
-- 
2.41.0