svn commit: r320981 - stable/11/sys/sys

[Konstantin Belousov]

Author: kib
Date: Fri Jul 14 07:42:57 2017
New Revision: 320981
URL: https://svnweb.freebsd.org/changeset/base/320981

Log:
  MFC r320501:
  Correct fences for sys/refcount.h.

Modified:
  stable/11/sys/sys/refcount.h
Directory Properties:
  stable/11/   (props changed)

Modified: stable/11/sys/sys/refcount.h
==============================================================================
--- stable/11/sys/sys/refcount.h	Fri Jul 14 02:15:48 2017	(r320980)
+++ stable/11/sys/sys/refcount.h	Fri Jul 14 07:42:57 2017	(r320981)
@@ -50,7 +50,7 @@ refcount_acquire(volatile u_int *count)
 {
 
 	KASSERT(*count < UINT_MAX, ("refcount %p overflowed", count));
-	atomic_add_acq_int(count, 1);	
+	atomic_add_int(count, 1);
 }
 
 static __inline int
@@ -58,10 +58,20 @@ refcount_release(volatile u_int *count)
 {
 	u_int old;
 
-	/* XXX: Should this have a rel membar? */
+	atomic_thread_fence_rel();
 	old = atomic_fetchadd_int(count, -1);
 	KASSERT(old > 0, ("negative refcount %p", count));
-	return (old == 1);
+	if (old > 1)
+		return (0);
+
+	/*
+	 * Last reference.  Signal the user to call the destructor.
+	 *
+	 * Ensure that the destructor sees all updates.  The fence_rel
+	 * at the start of the function synchronized with this fence.
+	 */
+	atomic_thread_fence_acq();
+	return (1);
 }
 
 #endif	/* ! __SYS_REFCOUNT_H__ */