Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
in i915_pmu_event_read. 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: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_pmu.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index d35973b41186..108b675088ba 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -696,12 +696,11 @@ static void i915_pmu_event_read(struct perf_event *event)
event->hw.state = PERF_HES_STOPPED;
return;
}
-again:
- prev = local64_read(&hwc->prev_count);
- new = __i915_pmu_event_read(event);
- if (local64_cmpxchg(&hwc->prev_count, prev, new) != prev)
- goto again;
+ prev = local64_read(&hwc->prev_count);
+ do {
+ new = __i915_pmu_event_read(event);
+ } while (!local64_try_cmpxchg(&hwc->prev_count, &prev, new));
local64_add(new - prev, &event->count);
}
--
2.41.0
