svn commit: r358007 - in head/sys: kern sys

[Mateusz Guzik]

Author: mjg
Date: Sun Feb 16 18:20:09 2020
New Revision: 358007
URL: https://svnweb.freebsd.org/changeset/base/358007

Log:
  refcount: update comments about fencing when releasing counts after r357989
  
  Requested by:	kib
  Reviewed by:	kib
  Differential Revision:	https://reviews.freebsd.org/D23719

Modified:
  head/sys/kern/kern_synch.c
  head/sys/sys/refcount.h

Modified: head/sys/kern/kern_synch.c
==============================================================================
--- head/sys/kern/kern_synch.c	Sun Feb 16 17:55:32 2020	(r358006)
+++ head/sys/kern/kern_synch.c	Sun Feb 16 18:20:09 2020	(r358007)
@@ -368,8 +368,8 @@ refcount_release_last(volatile u_int *count, u_int n, 
 	/*
 	 * Last reference.  Signal the user to call the destructor.
 	 *
-	 * Ensure that the destructor sees all updates.  The fence_rel
-	 * at the start of refcount_releasen synchronizes with this fence.
+	 * Ensure that the destructor sees all updates. This synchronizes
+	 * with release fences from all routines which drop the count.
 	 */
 	atomic_thread_fence_acq();
 	return (true);

Modified: head/sys/sys/refcount.h
==============================================================================
--- head/sys/sys/refcount.h	Sun Feb 16 17:55:32 2020	(r358006)
+++ head/sys/sys/refcount.h	Sun Feb 16 18:20:09 2020	(r358007)
@@ -119,6 +119,9 @@ refcount_releasen(volatile u_int *count, u_int n)
 	KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
 	    ("refcount_releasen: n=%u too large", n));
 
+	/*
+	 * Paired with acquire fence in refcount_release_last.
+	 */
 	atomic_thread_fence_rel();
 	old = atomic_fetchadd_int(count, -n);
 	if (__predict_false(n >= REFCOUNT_COUNT(old) ||
@@ -198,6 +201,9 @@ refcount_release_if_gt(volatile u_int *count, u_int n)
 			return (false);
 		if (__predict_false(REFCOUNT_SATURATED(old)))
 			return (true);
+		/*
+		 * Paired with acquire fence in refcount_release_last.
+		 */
 		if (atomic_fcmpset_rel_int(count, &old, old - 1))
 			return (true);
 	}