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@xxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: Vinod Koul <vkoul@xxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
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
