Merge tag 'locking-core-2024-03-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking updates from Ingo Molnar:

 - Micro-optimize local_xchg() and the rtmutex code on x86

 - Fix percpu-rwsem contention tracepoints

 - Simplify debugging Kconfig dependencies

 - Update/clarify the documentation of atomic primitives

 - Misc cleanups

* tag 'locking-core-2024-03-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/rtmutex: Use try_cmpxchg_relaxed() in mark_rt_mutex_waiters()
  locking/x86: Implement local_xchg() using CMPXCHG without the LOCK prefix
  locking/percpu-rwsem: Trigger contention tracepoints only if contended
  locking/rwsem: Make DEBUG_RWSEMS and PREEMPT_RT mutually exclusive
  locking/rwsem: Clarify that RWSEM_READER_OWNED is just a hint
  locking/mutex: Simplify <linux/mutex.h>
  locking/qspinlock: Fix 'wait_early' set but not used warning
  locking/atomic: scripts: Clarify ordering of conditional atomics

1 files changed, 14 insertions, 2 deletions

diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
index 73dba8b94443..59aa966dc212 100644
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -131,8 +131,20 @@ static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
 				 (typeof(l->a.counter) *) old, new);
 }
 
-/* Always has a lock prefix */
-#define local_xchg(l, n) (xchg(&((l)->a.counter), (n)))
+/*
+ * Implement local_xchg using CMPXCHG instruction without the LOCK prefix.
+ * XCHG is expensive due to the implied LOCK prefix.  The processor
+ * cannot prefetch cachelines if XCHG is used.
+ */
+static __always_inline long
+local_xchg(local_t *l, long n)
+{
+	long c = local_read(l);
+
+	do { } while (!local_try_cmpxchg(l, &c, n));
+
+	return c;
+}
 
 /**
  * local_add_unless - add unless the number is already a given value