[src/trunk]: src/sys/external/bsd/drm2/include/linux RCU pointer fixes.

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

details:   https://anonhg.NetBSD.org/src/rev/e97df2cae972
branches:  trunk
changeset: 1027930:e97df2cae972
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Dec 19 01:18:02 2021 +0000

description:
RCU pointer fixes.

- Use __typeof__, not typeof.
- Use membar_exit, not membar_producer.
  => We want a store-release, not store/store barrier.
  => Consider:
       foo->x = 42;
       assert(foo->x == 42);
       rcu_assign_pointer(globalfoo, foo);
     The foo->x load can be reordered past the store/store barrier
     (membar_producer), and happen after another thread has clobbered
     foo->x.
- Put the evaluation of the right-hand side before the barrier.

diffstat:

 sys/external/bsd/drm2/include/linux/rcupdate.h |  9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diffs (26 lines):

diff -r fb9ae7d2dd13 -r e97df2cae972 sys/external/bsd/drm2/include/linux/rcupdate.h
--- a/sys/external/bsd/drm2/include/linux/rcupdate.h    Sun Dec 19 01:17:54 2021 +0000
+++ b/sys/external/bsd/drm2/include/linux/rcupdate.h    Sun Dec 19 01:18:02 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: rcupdate.h,v 1.8 2021/12/19 01:17:54 riastradh Exp $   */
+/*     $NetBSD: rcupdate.h,v 1.9 2021/12/19 01:18:02 riastradh Exp $   */
 
 /*-
  * Copyright (c) 2018 The NetBSD Foundation, Inc.
@@ -41,12 +41,13 @@
 #define        RCU_INIT_POINTER(P, V)  ((P) = (V))
 
 #define        rcu_assign_pointer(P, V) do {                                         \
-       membar_producer();                                                    \
-       (P) = (V);                                                            \
+       __typeof__(*(P)) *__rcu_assign_pointer_tmp = (V);                     \
+       membar_exit();                                                        \
+       (P) = __rcu_assign_pointer_tmp;                                       \
 } while (0)
 
 #define        rcu_dereference(P) ({                                                 \
-       typeof(*(P)) *__rcu_dereference_tmp = (P);                            \
+       __typeof__(*(P)) *__rcu_dereference_tmp = (P);                        \
        membar_datadep_consumer();                                            \
        __rcu_dereference_tmp;                                                \
 })