From 2fa2ea233bec906b682fc82376649a1a6e18e9df Mon Sep 17 00:00:00 2001

From: "H.J. Lu" <hjl.tools@gmail.com>

Date: Tue, 2 Nov 2021 18:33:07 -0700

Subject: [PATCH] Add LLL_MUTEX_READ_LOCK [BZ #28537]

CAS instruction is expensive.  From the x86 CPU's point of view, getting

a cache line for writing is more expensive than reading.  See Appendix

A.2 Spinlock in:

https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf

The full compare and swap will grab the cache line exclusive and cause

excessive cache line bouncing.

Add LLL_MUTEX_READ_LOCK to do an atomic load and skip CAS in spinlock

loop if compare may fail to reduce cache line bouncing on contended locks.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

(cherry picked from commit d672a98a1af106bd68deb15576710cd61363f7a6)

---

 nptl/pthread_mutex_lock.c | 7 +++++++

 1 file changed, 7 insertions(+)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c

index 60ada70d..eb4d8baa 100644

--- a/nptl/pthread_mutex_lock.c

+++ b/nptl/pthread_mutex_lock.c

@@ -56,6 +56,11 @@

 #define FORCE_ELISION(m, s)

 #endif

+#ifndef LLL_MUTEX_READ_LOCK

+# define LLL_MUTEX_READ_LOCK(mutex) \

+  atomic_load_relaxed (&(mutex)->__data.__lock)

+#endif

+

 static int __pthread_mutex_lock_full (pthread_mutex_t *mutex)

      __attribute_noinline__;

@@ -136,6 +141,8 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)

 		  break;

 		}

 	      atomic_spin_nop ();

+	      if (LLL_MUTEX_READ_LOCK (mutex) != 0)

+		continue;

 	    }

 	  while (LLL_MUTEX_TRYLOCK (mutex) != 0);

-- 

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