compiler.h: Remove data_race() and unnecessary checks from {READ,WRITE}_ONCE()

The volatile accesses no longer need to be wrapped in data_race()
because compilers that emit instrumentation distinguishing volatile
accesses are required for KCSAN.

Consequently, the explicit kcsan_check_atomic*() are no longer required
either since the compiler emits instrumentation distinguishing the
volatile accesses.

Finally, simplify __READ_ONCE_SCALAR() and remove __WRITE_ONCE_SCALAR().

 [ bp: Convert commit message to passive voice. ]
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200521142047.169334-9-elver@google.com

include/linux/compiler.h

@@ -281,9 +281,7 @@ 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;							\
 })
@@ -299,17 +297,10 @@ 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