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From: Sunil Pandey <>
Subject: [glibc/release/2.34/master] Add LLL_MUTEX_READ_LOCK [BZ #28537]
Date: Wed, 28 Sep 2022 16:06:04 +0000 (GMT)	[thread overview]
Message-ID: <> (raw);h=a6b81f605dfba8650ea1f80122f41eb8e6c73dc7

commit a6b81f605dfba8650ea1f80122f41eb8e6c73dc7
Author: H.J. Lu <>
Date:   Tue Nov 2 18:33:07 2021 -0700

    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:
    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 <>
    (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 a04e015845..9f40928cc6 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -65,6 +65,11 @@ lll_mutex_lock_optimized (pthread_mutex_t *mutex)
+# define LLL_MUTEX_READ_LOCK(mutex) \
+  atomic_load_relaxed (&(mutex)->__data.__lock)
 static int __pthread_mutex_lock_full (pthread_mutex_t *mutex)
@@ -142,6 +147,8 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
 	      atomic_spin_nop ();
+	      if (LLL_MUTEX_READ_LOCK (mutex) != 0)
+		continue;
 	  while (LLL_MUTEX_TRYLOCK (mutex) != 0);

                 reply	other threads:[~2022-09-28 16:06 UTC|newest]

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