[PATCH] READ_ONCE, WRITE_ONCE, kcsan: Perform checks in __*_ONCE variants

[Marco Elver]

If left plain, using __READ_ONCE and __WRITE_ONCE will result in many
false positives with KCSAN due to being instrumented normally. To fix,
we should move the kcsan_check and data_race into __*_ONCE.

Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
A proposal to fix the problem with __READ_ONCE/__WRITE_ONCE and KCSAN
false positives.

Will, please feel free to take this patch and fiddle with it until it
looks like what you want if this is completely off.

Note: Currently __WRITE_ONCE_SCALAR seems to serve no real purpose. Do
we still need it?
---
 include/linux/compiler.h | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 741c93c62ecf..e902ca5de811 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -224,13 +224,16 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * atomicity or dependency ordering guarantees. Note that this may result
  * in tears!
  */
-#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
+#define __READ_ONCE(x)							\
+({									\
+	kcsan_check_atomic_read(&(x), sizeof(x));			\
+	data_race((*(const volatile __unqual_scalar_typeof(x) *)&(x))); \
+})
 
 #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(*__xp);		\
 	smp_read_barrier_depends();					\
 	(typeof(x))__x;							\
 })
@@ -243,14 +246,14 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 
 #define __WRITE_ONCE(x, val)						\
 do {									\
-	*(volatile typeof(x) *)&(x) = (val);				\
+	kcsan_check_atomic_write(&(x), sizeof(x));			\
+	data_race(*(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; }));			\
+	__WRITE_ONCE(*__xp, val);					\
 } while (0)
 
 #define WRITE_ONCE(x, val)						\
-- 
2.26.2.645.ge9eca65c58-goog