From: Uros Bizjak Date: Thu, 14 Aug 2025 06:43:26 +0000 (+0100) Subject: drm/i915/active: Use try_cmpxchg64() in __active_lookup() X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=60b006b7cf0dd5e8ee99bd77fb4573e1a3956035;p=users%2Fhch%2Fmisc.git 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 Cc: Jani Nikula Cc: Joonas Lahtinen Cc: Rodrigo Vivi Cc: Tvrtko Ursulin Cc: David Airlie Cc: Simona Vetter Reviewed-by: Tvrtko Ursulin Signed-off-by: Tvrtko Ursulin Signed-off-by: Tvrtko Ursulin Link: https://lore.kernel.org/r/20250814064326.95519-1-tvrtko.ursulin@igalia.com --- diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c index 0dbc4e289300..6b0c1162505a 100644 --- a/drivers/gpu/drm/i915/i915_active.c +++ b/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; }