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From: Sunil Pandey <skpgkp2@sourceware.org> To: glibc-cvs@sourceware.org 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: <20220928160604.5ECC33857413@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=a6b81f605dfba8650ea1f80122f41eb8e6c73dc7 commit a6b81f605dfba8650ea1f80122f41eb8e6c73dc7 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> (cherry picked from commit d672a98a1af106bd68deb15576710cd61363f7a6) 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 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 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__; @@ -142,6 +147,8 @@ 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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