[src/trunk]: src/sys/arch/alpha/alpha alpha: Fix placement of membar for send...

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

details:   https://anonhg.NetBSD.org/src/rev/c604c42641f5
branches:  trunk
changeset: 362429:c604c42641f5
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Feb 27 14:17:10 2022 +0000

description:
alpha: Fix placement of membar for sending ICCB message.

While here, reduce it to membar_exit -- it's obviously not needed for
store-before-load here (although alpha doesn't have anything weaker
than the full sequential consistency `mb'), and although we do need a
store-before-load (and load-before-load) to spin waiting for the CPU
to wake up, that already happens a few lines below with alpha_mb in
the loop anyway.  So no need for membar_sync, which is just `mb'
under the hood -- deleting the membar_sync in this place can't hurt.

The membar_sync had been inserted automatically when converting from
an older style of atomic_ops(3) API.

diffstat:

 sys/arch/alpha/alpha/cpu.c |  9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diffs (34 lines):

diff -r 3d914fbef052 -r c604c42641f5 sys/arch/alpha/alpha/cpu.c
--- a/sys/arch/alpha/alpha/cpu.c        Sun Feb 27 14:16:43 2022 +0000
+++ b/sys/arch/alpha/alpha/cpu.c        Sun Feb 27 14:17:10 2022 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: cpu.c,v 1.104 2021/05/05 03:54:16 thorpej Exp $ */
+/* $NetBSD: cpu.c,v 1.105 2022/02/27 14:17:10 riastradh Exp $ */
 
 /*-
  * Copyright (c) 1998, 1999, 2000, 2001, 2020 The NetBSD Foundation, Inc.
@@ -59,7 +59,7 @@
 
 #include <sys/cdefs.h>                 /* RCS ID & Copyright macro defns */
 
-__KERNEL_RCSID(0, "$NetBSD: cpu.c,v 1.104 2021/05/05 03:54:16 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: cpu.c,v 1.105 2022/02/27 14:17:10 riastradh Exp $");
 
 #include "opt_ddb.h"
 #include "opt_multiprocessor.h"
@@ -882,12 +882,13 @@
 
        /*
         * Copy the message into the ICCB, and tell the secondary console
-        * that it's there.
+        * that it's there.  Ensure the buffer is initialized before we
+        * set the rxrdy bits, as a store-release.
         */
        strcpy(pcsp->pcs_iccb.iccb_rxbuf, msg);
        pcsp->pcs_iccb.iccb_rxlen = strlen(msg);
+       membar_exit();
        atomic_or_ulong(&hwrpb->rpb_rxrdy, cpumask);
-       membar_sync();
 
        /* Wait for the message to be received. */
        for (timeout = 10000; timeout != 0; timeout--) {