[PATCH v6 1/4] Add LLL_MUTEX_READ_LOCK [BZ #28537]
H.J. Lu
hjl.tools@gmail.com
Thu Nov 11 16:24:25 GMT 2021
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.
---
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 2bd41767e0..72058c719c 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -64,6 +64,11 @@ lll_mutex_lock_optimized (pthread_mutex_t *mutex)
# define PTHREAD_MUTEX_VERSIONS 1
#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__;
@@ -141,6 +146,8 @@ 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);
--
2.33.1
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