[glibc] nptl: Effectively skip CAS in spinlock loop

[glibc] nptl: Effectively skip CAS in spinlock loop

[H.J. Lu]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6b8dbbd03ac88f169b65b5c7d7278576a11d2e44

commit 6b8dbbd03ac88f169b65b5c7d7278576a11d2e44
Author: Jangwoong Kim <6812skiii@gmail.com>
Date:   Tue Dec 14 21:30:51 2021 +0900

    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:
---
 nptl/pthread_mutex_lock.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index 31dfc7f1d7..d2e652d151 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;
 	}