[src/trunk]: src/sys/kern pcq(9): Explain why membar_release isn't needed in ...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/fc12a1ddd3cb
branches:  trunk
changeset: 373658:fc12a1ddd3cb
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Thu Feb 23 03:01:49 2023 +0000

description:
pcq(9): Explain why membar_release isn't needed in pcq_get.

No functional change intended.

diffstat:

 sys/kern/subr_pcq.c |  14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

diffs (37 lines):

diff -r bff767b7127c -r fc12a1ddd3cb sys/kern/subr_pcq.c
--- a/sys/kern/subr_pcq.c       Thu Feb 23 03:01:35 2023 +0000
+++ b/sys/kern/subr_pcq.c       Thu Feb 23 03:01:49 2023 +0000
@@ -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 @@
        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();