Fix caller information for warn_slowpath_null

Ian Campbell noticed that since "Eliminate thousands of warnings with
gcc 3.2 build" (commit 57adc4d2dbf968fdbe516359688094eef4d46581) all
WARN_ON()'s currently appear to come from warn_slowpath_null(), eg:

  WARNING: at kernel/softirq.c:143 warn_slowpath_null+0x1c/0x20()

because now that warn_slowpath_null() is in the call path, the
__builtin_return_address(0) returns that, rather than the place that
caused the warning.

Fix this by splitting up the warn_slowpath_null/fmt cases differently,
using a common helper function, and getting the return address in the
right place.  This also happens to avoid the unnecessary stack usage for
the non-stdargs case, and just generally cleans things up.

Make the function name printout use %pS while at it.

Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/panic.c

@@ -340,39 +340,44 @@ void oops_exit(void)
 }
 
 #ifdef WANT_WARN_ON_SLOWPATH
-void warn_slowpath_fmt(const char *file, int line, const char *fmt, ...)
-{
+struct slowpath_args {
+	const char *fmt;
 	va_list args;
-	char function[KSYM_SYMBOL_LEN];
-	unsigned long caller = (unsigned long)__builtin_return_address(0);
-	const char *board;
+};
 
-	sprint_symbol(function, caller);
+static void warn_slowpath_common(const char *file, int line, void *caller, struct slowpath_args *args)
+{
+	const char *board;
 
 	printk(KERN_WARNING "------------[ cut here ]------------\n");
-	printk(KERN_WARNING "WARNING: at %s:%d %s()\n", file,
-		line, function);
+	printk(KERN_WARNING "WARNING: at %s:%d %pS()\n", file, line, caller);
 	board = dmi_get_system_info(DMI_PRODUCT_NAME);
 	if (board)
 		printk(KERN_WARNING "Hardware name: %s\n", board);
 
-	if (*fmt) {
-		va_start(args, fmt);
-		vprintk(fmt, args);
-		va_end(args);
-	}
+	if (args)
+		vprintk(args->fmt, args->args);
 
 	print_modules();
 	dump_stack();
 	print_oops_end_marker();
 	add_taint(TAINT_WARN);
 }
+
+void warn_slowpath_fmt(const char *file, int line, const char *fmt, ...)
+{
+	struct slowpath_args args;
+
+	args.fmt = fmt;
+	va_start(args.args, fmt);
+	warn_slowpath_common(file, line, __builtin_return_address(0), &args);
+	va_end(args.args);
+}
 EXPORT_SYMBOL(warn_slowpath_fmt);
 
 void warn_slowpath_null(const char *file, int line)
 {
-	static const char *empty = "";
-	warn_slowpath_fmt(file, line, empty);
+	warn_slowpath_common(file, line, __builtin_return_address(0), NULL);
 }
 EXPORT_SYMBOL(warn_slowpath_null);
 #endif