47 lines
1.6 KiB
Diff
47 lines
1.6 KiB
Diff
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commit a6b81f605dfba8650ea1f80122f41eb8e6c73dc7
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Author: H.J. Lu <hjl.tools@gmail.com>
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Date: Tue Nov 2 18:33:07 2021 -0700
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Add LLL_MUTEX_READ_LOCK [BZ #28537]
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CAS instruction is expensive. From the x86 CPU's point of view, getting
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a cache line for writing is more expensive than reading. See Appendix
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A.2 Spinlock in:
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https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf
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The full compare and swap will grab the cache line exclusive and cause
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excessive cache line bouncing.
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Add LLL_MUTEX_READ_LOCK to do an atomic load and skip CAS in spinlock
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loop if compare may fail to reduce cache line bouncing on contended locks.
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Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
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(cherry picked from commit d672a98a1af106bd68deb15576710cd61363f7a6)
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diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
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index a04e0158451c8fff..9f40928cc6b9a067 100644
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--- a/nptl/pthread_mutex_lock.c
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+++ b/nptl/pthread_mutex_lock.c
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@@ -65,6 +65,11 @@ lll_mutex_lock_optimized (pthread_mutex_t *mutex)
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# define PTHREAD_MUTEX_VERSIONS 1
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#endif
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+#ifndef LLL_MUTEX_READ_LOCK
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+# define LLL_MUTEX_READ_LOCK(mutex) \
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+ atomic_load_relaxed (&(mutex)->__data.__lock)
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+#endif
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+
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static int __pthread_mutex_lock_full (pthread_mutex_t *mutex)
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__attribute_noinline__;
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@@ -142,6 +147,8 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex)
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break;
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}
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atomic_spin_nop ();
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+ if (LLL_MUTEX_READ_LOCK (mutex) != 0)
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+ continue;
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}
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while (LLL_MUTEX_TRYLOCK (mutex) != 0);
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