drm/i915/active: Use try_cmpxchg64() in __active_lookup()

Replace this pattern in __active_lookup():

    cmpxchg64(*ptr, old, new) == old

... with the simpler and faster:

    try_cmpxchg64(*ptr, &old, new)

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after the CMPXCHG.

The patch also improves the explanation of what the code really
does. cmpxchg64() will *succeed* for the winner of the race and
try_cmpxchg64() nicely documents this fact.

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Simona Vetter <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Signed-off-by: Tvrtko Ursulin <[email protected]>
Signed-off-by: Tvrtko Ursulin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ubizjak

drivers/gpu/drm/i915/i915_active.c

@@ -257,10 +257,9 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
 		 * claimed the cache and we know that is does not match our
 		 * idx. If, and only if, the timeline is currently zero is it
 		 * worth competing to claim it atomically for ourselves (for
-		 * only the winner of that race will cmpxchg return the old
-		 * value of 0).
+		 * only the winner of that race will cmpxchg succeed).
 		 */
-		if (!cached && !cmpxchg64(&it->timeline, 0, idx))
+		if (!cached && try_cmpxchg64(&it->timeline, &cached, idx))
 			return it;
 	}