perf/x86/rapl: Use local64_try_cmpxchg in rapl_event_update()

Use local64_try_cmpxchg() instead of local64_cmpxchg(*ptr, old, new) == old.

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.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ubizjak

arch/x86/events/rapl.c

@@ -179,13 +179,11 @@ static u64 rapl_event_update(struct perf_event *event)
 	s64 delta, sdelta;
 	int shift = RAPL_CNTR_WIDTH;
 
-again:
 	prev_raw_count = local64_read(&hwc->prev_count);
-	rdmsrl(event->hw.event_base, new_raw_count);
-
-	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
-			    new_raw_count) != prev_raw_count)
-		goto again;
+	do {
+		rdmsrl(event->hw.event_base, new_raw_count);
+	} while (!local64_try_cmpxchg(&hwc->prev_count,
+				      &prev_raw_count, new_raw_count));
 
 	/*
 	 * Now we have the new raw value and have updated the prev