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* [glibc] Add LLL_MUTEX_READ_LOCK [BZ #28537]
@ 2021-11-12 18:32 H.J. Lu
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From: H.J. Lu @ 2021-11-12 18:32 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=d672a98a1af106bd68deb15576710cd61363f7a6
commit d672a98a1af106bd68deb15576710cd61363f7a6
Author: H.J. Lu <hjl.tools@gmail.com>
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:
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>
Diff:
---
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 59a28cff1a..762059b230 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);
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