kcsan: Add __no_kcsan function attribute

Since the use of -fsanitize=thread is an implementation detail of KCSAN,
the name __no_sanitize_thread could be misleading if used widely.
Instead, we introduce the __no_kcsan attribute which is shorter and more
accurate in the context of KCSAN.

This matches the attribute name __no_kcsan_or_inline. The use of
__kcsan_or_inline itself is still required for __always_inline functions
to retain compatibility with older compilers.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/compiler-gcc.h

@@ -146,8 +146,7 @@
 #endif
 
 #if defined(__SANITIZE_THREAD__) && __has_attribute(__no_sanitize_thread__)
-#define __no_sanitize_thread                                                   \
-	__attribute__((__noinline__)) __attribute__((no_sanitize_thread))
+#define __no_sanitize_thread __attribute__((no_sanitize_thread))
 #else
 #define __no_sanitize_thread
 #endif

include/linux/compiler.h

@@ -207,12 +207,15 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define __no_kasan_or_inline __always_inline
 #endif
 
+#define __no_kcsan __no_sanitize_thread
 #ifdef __SANITIZE_THREAD__
 /*
  * Rely on __SANITIZE_THREAD__ instead of CONFIG_KCSAN, to avoid not inlining in
- * compilation units where instrumentation is disabled.
+ * compilation units where instrumentation is disabled. The attribute 'noinline'
+ * is required for older compilers, where implicit inlining of very small
+ * functions renders __no_sanitize_thread ineffective.
  */
-# define __no_kcsan_or_inline __no_sanitize_thread notrace __maybe_unused
+# define __no_kcsan_or_inline __no_kcsan noinline notrace __maybe_unused
 # define __no_sanitize_or_inline __no_kcsan_or_inline
 #else
 # define __no_kcsan_or_inline __always_inline