From: Jangwoong Kim <6812skiii@gmail.com>
Date: Tue, 14 Dec 2021 12:30:51 +0000 (+0900)
Subject: nptl: Effectively skip CAS in spinlock loop
X-Git-Tag: glibc-2.35~54
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=6b8dbbd03ac88f169b65b5c7d7278576a11d2e44;p=thirdparty%2Fglibc.git

nptl: Effectively skip CAS in spinlock loop

The commit:
"Add LLL_MUTEX_READ_LOCK [BZ #28537]"
SHA1: d672a98a1af106bd68deb15576710cd61363f7a6

introduced LLL_MUTEX_READ_LOCK, to skip CAS in spinlock loop
if atomic load fails. But, "continue" inside of do-while loop
does not skip the evaluation of escape expression, thus CAS
is not skipped.

Replace do-while with while and skip LLL_MUTEX_TRYLOCK if
LLL_MUTEX_READ_LOCK fails.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
---

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index 31dfc7f1d7d..d2e652d151d 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -146,10 +146,9 @@ 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);
+	  while (LLL_MUTEX_READ_LOCK (mutex) != 0
+		 || LLL_MUTEX_TRYLOCK (mutex) != 0);
 
 	  mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
 	}