Add memory barriers to libc's _spinlock() to make the mechanism

serialize memory accesses properly.

_spinlock()'s backend, _atomic_lock(), already issues an entry barrier
on some architectures, but that practice has not been consistent. This
patch generalizes the barrier use.

OK kettenis@, mpi@

1 files changed, 11 insertions, 2 deletions

diff --git a/lib/libc/thread/rthread.c b/lib/libc/thread/rthread.c
index f26e85ffd31..c61d8fd6f03 100644
--- a/lib/libc/thread/rthread.c
+++ b/lib/libc/thread/rthread.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: rthread.c,v 1.7 2017/12/05 13:45:31 kettenis Exp $ */
+/*	$OpenBSD: rthread.c,v 1.8 2018/05/13 16:21:26 visa Exp $ */
 /*
  * Copyright (c) 2004,2005 Ted Unangst <tedu@openbsd.org>
  * All Rights Reserved.
@@ -19,6 +19,9 @@
  * The infrastructure of rthreads
  */
 
+#include <sys/types.h>
+#include <sys/atomic.h>
+
 #include <pthread.h>
 #include <stdlib.h>
 #include <tib.h>
@@ -45,18 +48,24 @@ _spinlock(volatile _atomic_lock_t *lock)
 {
 	while (_atomic_lock(lock))
 		sched_yield();
+	membar_enter_after_atomic();
 }
 DEF_STRONG(_spinlock);
 
 int
 _spinlocktry(volatile _atomic_lock_t *lock)
 {
-	return 0 == _atomic_lock(lock);
+	if (_atomic_lock(lock) == 0) {
+		membar_enter_after_atomic();
+		return 1;
+	}
+	return 0;
 }
 
 void
 _spinunlock(volatile _atomic_lock_t *lock)
 {
+	membar_exit();
 	*lock = _ATOMIC_LOCK_UNLOCKED;
 }
 DEF_STRONG(_spinunlock);