From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 7852) id 5ECC33857413; Wed, 28 Sep 2022 16:06:04 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5ECC33857413 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1664381164; bh=mT3NjZGx4pnhCCnh5737NxnZJOrV8m5rKq0ftlY81SY=; h=From:To:Subject:Date:From; b=PjqHmqzcsGArFGKmkmQx1Cg3sgMlK1chFxfGyIyz4caFKJ5qsUlhm9HL9DLvprEv1 gMxv2hraZ+JSb8Rlp/nAAdSjz9JdMPTOQtaexG+Hue4KjeAfm85PTf7cWWgjvVSwme KGCtpEAFhaACgRZ/1GQE7/eXRCIvY5s+Huz00TFM= Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit From: Sunil Pandey To: glibc-cvs@sourceware.org Subject: [glibc/release/2.34/master] Add LLL_MUTEX_READ_LOCK [BZ #28537] X-Act-Checkin: glibc X-Git-Author: H.J. Lu X-Git-Refname: refs/heads/release/2.34/master X-Git-Oldrev: ed8300c054cae4aeb0bbfa043f5fccc91a4adbf5 X-Git-Newrev: a6b81f605dfba8650ea1f80122f41eb8e6c73dc7 Message-Id: <20220928160604.5ECC33857413@sourceware.org> Date: Wed, 28 Sep 2022 16:06:04 +0000 (GMT) List-Id: https://sourceware.org/git/gitweb.cgi?p=glibc.git;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: 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 (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);