[src/trunk]: src/sys/arch/riscv/riscv riscv: Optimization: Omit needless memb...

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

details:   https://anonhg.NetBSD.org/src/rev/deadb213fa5d
branches:  trunk
changeset: 373792:deadb213fa5d
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Wed Mar 01 08:18:24 2023 +0000

description:
riscv: 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

diffstat:

 sys/arch/riscv/riscv/cpu_switch.S |  10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diffs (24 lines):

diff -r abafeaf4d5f1 -r deadb213fa5d sys/arch/riscv/riscv/cpu_switch.S
--- a/sys/arch/riscv/riscv/cpu_switch.S Wed Mar 01 08:18:13 2023 +0000
+++ b/sys/arch/riscv/riscv/cpu_switch.S Wed Mar 01 08:18:24 2023 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: cpu_switch.S,v 1.3 2023/02/23 14:56:23 riastradh Exp $ */
+/* $NetBSD: cpu_switch.S,v 1.4 2023/03/01 08:18:24 riastradh Exp $ */
 
 /*-
  * Copyright (c) 2014 The NetBSD Foundation, Inc.
@@ -177,7 +177,13 @@
        mv      tp, a0                  // set thread pointer to newlwp
        fence   w,w                     // for mutex_enter; see cpu_switchto
        PTR_S   tp, CI_CURLWP(t1)       // update curlwp
-       fence   w,r                     // for mutex_enter; see cpu_switchto
+       /*
+        * 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.
+        */
        PTR_L   sp, L_MD_KTF(tp)        // switch to its stack
        csrw    sstatus, t0             // reenable interrupts
        call    _C_LABEL(softint_dispatch)