[patch resend 4.12] compiler, clang: always inline when CONFIG_OPTIMIZE_INLINING is disabled

[David Rientjes]

The motivation of commit abb2ea7dfd82 ("compiler, clang: suppress warning 
for unused static inline functions") is to suppress clang's warnings about 
unused static inline functions.

For configs without CONFIG_OPTIMIZE_INLINING enabled, such as any non-x86 
architecture, `inline' in the kernel implies that 
__attribute__((always_inline)) is used.

Some code depends on that behavior, see 
https://lkml.org/lkml/2017/6/13/918:

net/built-in.o: In function `__xchg_mb':
arch/arm64/include/asm/cmpxchg.h:99: undefined reference to `__compiletime_assert_99'
arch/arm64/include/asm/cmpxchg.h:99: undefined reference to `__compiletime_assert_99

The full fix would be to identify these breakages and annotate the 
functions with __always_inline instead of `inline'.  But since we are late 
in the 4.12-rc cycle, simply carry forward the forced inlining behavior 
and work toward moving arm64, and other architectures, toward 
CONFIG_OPTIMIZE_INLINING behavior.

Reported-by: Sodagudi Prasad <psodagud@xxxxxxxxxxxxxx>
Tested-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 Resend of http://marc.info/?l=linux-kernel&m=149681501816319 for 4.12
 inclusion.

 Prasad, please add your Tested-by.

 include/linux/compiler-clang.h |  8 --------
 include/linux/compiler-gcc.h   | 18 +++++++++++-------
 2 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -15,11 +15,3 @@
  * with any version that can compile the kernel
  */
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
-
-/*
- * GCC does not warn about unused static inline functions for
- * -Wunused-function.  This turns out to avoid the need for complex #ifdef
- * directives.  Suppress the warning in clang as well.
- */
-#undef inline
-#define inline inline __attribute__((unused)) notrace
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -66,18 +66,22 @@
 
 /*
  * Force always-inline if the user requests it so via the .config,
- * or if gcc is too old:
+ * or if gcc is too old.
+ * GCC does not warn about unused static inline functions for
+ * -Wunused-function.  This turns out to avoid the need for complex #ifdef
+ * directives.  Suppress the warning in clang as well by using "unused"
+ * function attribute, which is redundant but not harmful for gcc.
  */
 #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||		\
     !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
-#define inline		inline		__attribute__((always_inline)) notrace
-#define __inline__	__inline__	__attribute__((always_inline)) notrace
-#define __inline	__inline	__attribute__((always_inline)) notrace
+#define inline inline		__attribute__((always_inline,unused)) notrace
+#define __inline__ __inline__	__attribute__((always_inline,unused)) notrace
+#define __inline __inline	__attribute__((always_inline,unused)) notrace
 #else
 /* A lot of inline functions can cause havoc with function tracing */
-#define inline		inline		notrace
-#define __inline__	__inline__	notrace
-#define __inline	__inline	notrace
+#define inline inline		__attribute__((unused)) notrace
+#define __inline__ __inline__	__attribute__((unused)) notrace
+#define __inline __inline	__attribute__((unused)) notrace
 #endif
 
 #define __always_inline	inline __attribute__((always_inline))