[src/trunk]: src/sys/sys Add missing barriers to curlwp_bind and curlwp_bindx

[ozaki-r <ozaki-r%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/b3da3eafdde9
branches:  trunk
changeset: 829875:b3da3eafdde9
user:      ozaki-r <ozaki-r%NetBSD.org@localhost>
date:      Fri Feb 16 07:11:50 2018 +0000

description:
Add missing barriers to curlwp_bind and curlwp_bindx

The barriers prevent the instruction of setting/clearing the LP_BOUND flag
from reordering over where we want to prevent LWP migrations.

Note that the fix doesn't mean that there was a race condition.  For now the API
is used only for psref and the combination use of them doesn't need the
barriers(*).

(*) https://mail-index.netbsd.org/tech-kern/2018/02/15/msg023101.html

Pointed out by Mateusz Guzik

diffstat:

 sys/sys/lwp.h |  4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diffs (25 lines):

diff -r dfe8a94d2b07 -r b3da3eafdde9 sys/sys/lwp.h
--- a/sys/sys/lwp.h     Fri Feb 16 07:05:21 2018 +0000
+++ b/sys/sys/lwp.h     Fri Feb 16 07:11:50 2018 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: lwp.h,v 1.177 2018/01/14 16:43:03 maxv Exp $   */
+/*     $NetBSD: lwp.h,v 1.178 2018/02/16 07:11:50 ozaki-r Exp $        */
 
 /*
  * Copyright (c) 2001, 2006, 2007, 2008, 2009, 2010
@@ -536,6 +536,7 @@
 
        bound = curlwp->l_pflag & LP_BOUND;
        curlwp->l_pflag |= LP_BOUND;
+       __insn_barrier();
 
        return bound;
 }
@@ -545,6 +546,7 @@
 {
 
        KASSERT(curlwp->l_pflag & LP_BOUND);
+       __insn_barrier();
        curlwp->l_pflag ^= bound ^ LP_BOUND;
 }