[src/trunk]: src/sys/arch/arm/arm32 arm32: Optimization: Omit needless membar...

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

details:   https://anonhg.NetBSD.org/src/rev/4bfbe8b2695d
branches:  trunk
changeset: 373789:4bfbe8b2695d
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Wed Mar 01 08:17:53 2023 +0000

description:
arm32: Optimization: Omit needless membar when triggering softint.

When we are triggering a softint, it can't already hold any mutexes.
So any path to mutex_exit(mtx) must go via mutex_enter(mtx), which is
always done with atomic r/m/w, and we need not issue any explicit
barrier between ci->ci_curlwp = softlwp and a potential load of
mtx->mtx_owner in mutex_exit.

PR kern/57240

XXX pullup-8
XXX pullup-9
XXX pullup-10

diffstat:

 sys/arch/arm/arm32/cpuswitch.S |  14 +++++++++-----
 1 files changed, 9 insertions(+), 5 deletions(-)

diffs (35 lines):

diff -r 8375b74ce704 -r 4bfbe8b2695d sys/arch/arm/arm32/cpuswitch.S
--- a/sys/arch/arm/arm32/cpuswitch.S    Wed Mar 01 08:17:24 2023 +0000
+++ b/sys/arch/arm/arm32/cpuswitch.S    Wed Mar 01 08:17:53 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: cpuswitch.S,v 1.106 2023/02/23 14:55:25 riastradh Exp $        */
+/*     $NetBSD: cpuswitch.S,v 1.107 2023/03/01 08:17:53 riastradh Exp $        */
 
 /*
  * Copyright 2003 Wasabi Systems, Inc.
@@ -87,7 +87,7 @@
 #include <arm/asm.h>
 #include <arm/locore.h>
 
-       RCSID("$NetBSD: cpuswitch.S,v 1.106 2023/02/23 14:55:25 riastradh Exp $")
+       RCSID("$NetBSD: cpuswitch.S,v 1.107 2023/03/01 08:17:53 riastradh Exp $")
 
 /* LINTSTUB: include <sys/param.h> */
 
@@ -417,9 +417,13 @@
        dmb                             /* for mutex_enter; see cpu_switchto */
 #endif
        str     r5, [r7, #(CI_CURLWP)]  /* save new lwp */
-#ifdef _ARM_ARCH_7
-       dmb                             /* for mutex_enter; see cpu_switchto */
-#endif
+       /*
+        * No need for barrier after ci->ci_curlwp = softlwp -- when we
+        * enter a softint lwp, it can't be holding any mutexes, so it
+        * can't release any until after it has acquired them, so we
+        * need not participate in the protocol with mutex_vector_enter
+        * barriers here.
+        */
 
 #ifdef KASAN
        mov     r0, r5