From d8a84f08e87c0afe0e630de063e2a1198d02a8fd Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjg@FreeBSD.org>
Date: Sun, 16 Feb 2020 18:20:09 +0000
Subject: [PATCH] refcount: update comments about fencing when releasing counts
 after r357989

Requested by:	kib
Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D23719
---
 sys/kern/kern_synch.c | 4 ++--
 sys/sys/refcount.h    | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index 1a19e67bc21..435f1a63a52 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -368,8 +368,8 @@ refcount_release_last(volatile u_int *count, u_int n, u_int old)
 	/*
 	 * 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);
diff --git a/sys/sys/refcount.h b/sys/sys/refcount.h
index 9173793c328..a3becb5558a 100644
--- a/sys/sys/refcount.h
+++ b/sys/sys/refcount.h
@@ -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);
 	}