linux-kernel - [PATCH -tip v2 08/11] READ_ONCE, WRITE_ONCE: Remove data

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Message-Id: <20200521110854.114437-9-elver@google.com>
Date:   Thu, 21 May 2020 13:08:51 +0200
From:   Marco Elver <elver@...gle.com>
To:     elver@...gle.com
Cc:     paulmck@...nel.org, dvyukov@...gle.com, glider@...gle.com,
        andreyknvl@...gle.com, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        peterz@...radead.org, will@...nel.org,
        clang-built-linux@...glegroups.com, bp@...en8.de
Subject: [PATCH -tip v2 08/11] READ_ONCE, WRITE_ONCE: Remove data_race() and
 unnecessary checks

The volatile accesses no longer need to be wrapped in data_race(),
because we require compilers that emit instrumentation distinguishing
volatile accesses. Consequently, we also no longer require the explicit
kcsan_check_atomic*(), since the compiler emits instrumentation
distinguishing the volatile accesses. Finally, simplify
__READ_ONCE_SCALAR and remove __WRITE_ONCE_SCALAR.

Signed-off-by: Marco Elver <elver@...gle.com>
---
v2:
* Remove unnecessary kcsan_check_atomic*() in *_ONCE.
* Simplify __READ_ONCE_SCALAR and remove __WRITE_ONCE_SCALAR. This
  effectively restores Will Deacon's pre-KCSAN version:
  https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/tree/include/linux/compiler.h?h=rwonce/cleanup#n202
---
 include/linux/compiler.h | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 17c98b215572..7444f026eead 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -228,9 +228,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 
 #define __READ_ONCE_SCALAR(x)						\
 ({									\
-	typeof(x) *__xp = &(x);						\
-	__unqual_scalar_typeof(x) __x = data_race(__READ_ONCE(*__xp));	\
-	kcsan_check_atomic_read(__xp, sizeof(*__xp));			\
+	__unqual_scalar_typeof(x) __x = __READ_ONCE(x);			\
 	smp_read_barrier_depends();					\
 	(typeof(x))__x;							\
 })
@@ -246,17 +244,10 @@ do {									\
 	*(volatile typeof(x) *)&(x) = (val);				\
 } while (0)
 
-#define __WRITE_ONCE_SCALAR(x, val)					\
-do {									\
-	typeof(x) *__xp = &(x);						\
-	kcsan_check_atomic_write(__xp, sizeof(*__xp));			\
-	data_race(({ __WRITE_ONCE(*__xp, val); 0; }));			\
-} while (0)
-
 #define WRITE_ONCE(x, val)						\
 do {									\
 	compiletime_assert_rwonce_type(x);				\
-	__WRITE_ONCE_SCALAR(x, val);					\
+	__WRITE_ONCE(x, val);						\
 } while (0)
 
 #ifdef CONFIG_KASAN
-- 
2.26.2.761.g0e0b3e54be-goog