Index: pthread_mutex.c
===================================================================
RCS file: /cvsroot/src/lib/libpthread/pthread_mutex.c,v
retrieving revision 1.56
diff -p -u -r1.56 pthread_mutex.c
--- pthread_mutex.c	21 Mar 2013 16:49:12 -0000	1.56
+++ pthread_mutex.c	2 Feb 2014 19:58:49 -0000
@@ -311,9 +311,24 @@ pthread__mutex_lock_slow(pthread_mutex_t
 				 * sys/kern/kern_mutex.c for details.
 				 * In short, we must spin if we see
 				 * that the holder is running again.
+				 *
+				 * After we have observed the holder
+				 * not running, we must issue a
+				 * membar_consumer and reload
+				 * ptm_owner so that if the holder
+				 * wrote NULL to ptm_owner before it
+				 * was preempted, we will observe the
+				 * NULL write now and not some time
+				 * later when we have decided to sleep
+				 * with nobody around to wake us.
+				 * Preemption issues membar_producer
+				 * before it reports the thread not
+				 * running, which matches this
+				 * membar_consumer.
 				 */
-				membar_sync();
-				next = pthread__mutex_spin(ptm, owner);
+				(void)pthread__mutex_spin(ptm, owner);
+				membar_consumer();
+				next = ptm->ptm_owner;
 			}
 		}
 
Index: pthread_types.h
===================================================================
RCS file: /cvsroot/src/lib/libpthread/pthread_types.h,v
retrieving revision 1.13
diff -p -u -r1.13 pthread_types.h
--- pthread_types.h	2 Aug 2008 19:46:30 -0000	1.13
+++ pthread_types.h	2 Feb 2014 19:58:49 -0000
@@ -109,7 +109,7 @@ struct	__pthread_mutex_st {
 	uint8_t		ptm_pad2[3];
 #endif
 	volatile pthread_t ptm_owner;
-	pthread_t * volatile ptm_waiters;
+	volatile pthread_t ptm_waiters;
 	unsigned int	ptm_recursed;
 	void		*ptm_spare2;	/* unused - backwards compat */
 };