kcsan: Turn barrier instrumentation into macros

Some architectures use barriers in 'extern inline' functions, from which
we should not refer to static inline functions.

For example, building Alpha with gcc and W=1 shows:

./include/asm-generic/barrier.h:70:30: warning: 'kcsan_rmb' is static but used in inline function 'pmd_offset' which is not static
   70 | #define smp_rmb()       do { kcsan_rmb(); __smp_rmb(); } while (0)
      |                              ^~~~~~~~~
./arch/alpha/include/asm/pgtable.h:293:9: note: in expansion of macro 'smp_rmb'
  293 |         smp_rmb(); /* see above */
      |         ^~~~~~~

Which seems to warn about 6.7.4#3 of the C standard:
  "An inline definition of a function with external linkage shall not
   contain a definition of a modifiable object with static or thread
   storage duration, and shall not contain a reference to an identifier
   with internal linkage."

Fix it by turning barrier instrumentation into macros, which matches
definitions in <asm/barrier.h>.

Perhaps we can revert this change in future, when there are no more
'extern inline' users left.

Link: https://lkml.kernel.org/r/202112041334.X44uWZXf-lkp@intel.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/kcsan-checks.h

@@ -241,28 +241,30 @@ static inline void __kcsan_disable_current(void) { }
  * disabled with the __no_kcsan function attribute.
  *
  * Also see definition of __tsan_atomic_signal_fence() in kernel/kcsan/core.c.
+ *
+ * These are all macros, like <asm/barrier.h>, since some architectures use them
+ * in non-static inline functions.
  */
 #define __KCSAN_BARRIER_TO_SIGNAL_FENCE(name)					\
-	static __always_inline void kcsan_##name(void)				\
-	{									\
+	do {									\
 		barrier();							\
 		__atomic_signal_fence(__KCSAN_BARRIER_TO_SIGNAL_FENCE_##name);	\
 		barrier();							\
-	}
-__KCSAN_BARRIER_TO_SIGNAL_FENCE(mb)
-__KCSAN_BARRIER_TO_SIGNAL_FENCE(wmb)
-__KCSAN_BARRIER_TO_SIGNAL_FENCE(rmb)
-__KCSAN_BARRIER_TO_SIGNAL_FENCE(release)
+	} while (0)
+#define kcsan_mb()	__KCSAN_BARRIER_TO_SIGNAL_FENCE(mb)
+#define kcsan_wmb()	__KCSAN_BARRIER_TO_SIGNAL_FENCE(wmb)
+#define kcsan_rmb()	__KCSAN_BARRIER_TO_SIGNAL_FENCE(rmb)
+#define kcsan_release()	__KCSAN_BARRIER_TO_SIGNAL_FENCE(release)
 #elif defined(CONFIG_KCSAN_WEAK_MEMORY) && defined(__KCSAN_INSTRUMENT_BARRIERS__)
 #define kcsan_mb	__kcsan_mb
 #define kcsan_wmb	__kcsan_wmb
 #define kcsan_rmb	__kcsan_rmb
 #define kcsan_release	__kcsan_release
 #else /* CONFIG_KCSAN_WEAK_MEMORY && ... */
-static inline void kcsan_mb(void)		{ }
-static inline void kcsan_wmb(void)		{ }
-static inline void kcsan_rmb(void)		{ }
-static inline void kcsan_release(void)		{ }
+#define kcsan_mb()	do { } while (0)
+#define kcsan_wmb()	do { } while (0)
+#define kcsan_rmb()	do { } while (0)
+#define kcsan_release()	do { } while (0)
 #endif /* CONFIG_KCSAN_WEAK_MEMORY && ... */
 
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