Module Name: src
Committed By: riastradh
Date: Thu Feb 23 03:01:49 UTC 2023
Modified Files:
src/sys/kern: subr_pcq.c
Log Message:
pcq(9): Explain why membar_release isn't needed in pcq_get.
No functional change intended.
To generate a diff of this commit:
cvs rdiff -u -r1.16 -r1.17 src/sys/kern/subr_pcq.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/subr_pcq.c
diff -u src/sys/kern/subr_pcq.c:1.16 src/sys/kern/subr_pcq.c:1.17
--- src/sys/kern/subr_pcq.c:1.16 Thu Feb 23 03:01:35 2023
+++ src/sys/kern/subr_pcq.c Thu Feb 23 03:01:49 2023
@@ -1,4 +1,4 @@
-/* $NetBSD: subr_pcq.c,v 1.16 2023/02/23 03:01:35 riastradh Exp $ */
+/* $NetBSD: subr_pcq.c,v 1.17 2023/02/23 03:01:49 riastradh Exp $ */
/*-
* Copyright (c) 2009, 2019 The NetBSD Foundation, Inc.
@@ -34,7 +34,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_pcq.c,v 1.16 2023/02/23 03:01:35 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_pcq.c,v 1.17 2023/02/23 03:01:49 riastradh Exp $");
#include <sys/param.h>
#include <sys/types.h>
@@ -177,10 +177,16 @@ pcq_get(pcq_t *pcq)
nv = pcq_combine(p, c);
/*
- * Ensure that update to pcq_items[] becomes globally visible
+ * Ensure that update to pcq_items[c] becomes globally visible
* before the update to pcq_pc. If it were reordered to occur
* after it, we could in theory wipe out a modification made
- * to pcq_items[] by pcq_put().
+ * to pcq_items[c] by pcq_put().
+ *
+ * No need for load-before-store ordering of membar_release
+ * because the only load we need to ensure happens first is the
+ * load of pcq->pcq_items[c], but that necessarily happens
+ * before the store to pcq->pcq_items[c] to null it out because
+ * it is at the same memory location.
*/
#ifndef __HAVE_ATOMIC_AS_MEMBAR
membar_producer();
