public inbox for glibc-cvs@sourceware.org
help / color / mirror / Atom feed
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);

                 reply	other threads:[~2022-09-28 16:06 UTC|newest]

Thread overview: [no followups] expand[flat|nested]  mbox.gz  Atom feed

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20220928160604.5ECC33857413@sourceware.org \
    --to=skpgkp2@sourceware.org \
    --cc=glibc-cvs@sourceware.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).