MIPS: cmpxchg: Use __compiletime_error() for bad cmpxchg() pointers

Our cmpxchg() implementation relies upon generating a call to a function
which doesn't really exist (__cmpxchg_called_with_bad_pointer) to create
a link failure in cases where cmpxchg() is called with a pointer to a
value of an unsupported size.

The __compiletime_error macro can be used to decorate a function such
that a call to it generates a compile-time, rather than a link-time,
error. This patch uses __compiletime_error to cause bad cmpxchg() calls
to error out at compile time rather than link time, allowing errors to
occur more quickly & making it easier to spot where the problem comes
from.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16350/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/cmpxchg.h

@@ -137,10 +137,17 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
 })
 
 /*
- * This function doesn't exist, so you'll get a linker error
- * if something tries to do an invalid cmpxchg().
+ * This function doesn't exist, so if it is called you'll either:
+ *
+ * - Get an error at compile-time due to __compiletime_error, if supported by
+ *   your compiler.
+ *
+ * or:
+ *
+ * - Get an error at link-time due to the call to the missing function.
  */
-extern void __cmpxchg_called_with_bad_pointer(void);
+extern void __cmpxchg_called_with_bad_pointer(void)
+	__compiletime_error("Bad argument size for cmpxchg");
 
 #define __cmpxchg(ptr, old, new, pre_barrier, post_barrier)		\
 ({									\