markers: add an if(0) to __mark_check_format()

Wrap __mark_check_format() into an if(0) to make sure that parameters such as

trace_mark(mm_page_alloc, "order %u pfn %lu", order, page?page_to_pfn(page):0);

(where page_to_pfn() has side-effects) won't generate code because of the
__mark_check_format().

Thanks to Jan Kiszka for reporting this.
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: "Frank Ch. Eigler" <fche@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/marker.h

@@ -104,10 +104,16 @@ static inline void marker_update_probe_range(struct marker *begin,
 #define MARK_NOARGS " "
 
 /* To be used for string format validity checking with gcc */
-static inline void __printf(1, 2) __mark_check_format(const char *fmt, ...)
+static inline void __printf(1, 2) ___mark_check_format(const char *fmt, ...)
 {
 }
 
+#define __mark_check_format(format, args...)				\
+	do {								\
+		if (0)							\
+			___mark_check_format(format, ## args);		\
+	} while (0)
+
 extern marker_probe_func __mark_empty_function;
 
 extern void marker_probe_cb(const struct marker *mdata,