bug: set warn variable before calling WARN()

This has hit me a couple of times already.  I would be debugging code
and the system would simply hang and then reboot.  Finally, I found that
the problem was caused by WARN_ON_ONCE() and friends.

The macro WARN_ON_ONCE(condition) is defined as:

	static bool __section(.data.unlikely) __warned;
	int __ret_warn_once = !!(condition);

	if (unlikely(__ret_warn_once))
		if (WARN_ON(!__warned))
			__warned = true;

	unlikely(__ret_warn_once);

Which looks great and all.  But what I have hit, is an issue when
WARN_ON() itself hits the same WARN_ON_ONCE() code.  Because, the
variable __warned is not yet set.  Then it too calls WARN_ON() and that
triggers the warning again.  It keeps doing this until the stack is
overflowed and the system crashes.

By setting __warned first before calling WARN_ON() makes the original
WARN_ON_ONCE() really only warn once, and not an infinite amount of
times if the WARN_ON() also triggers the warning.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 630dd23..fdf6fa0 100644 (file)
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -110,9 +110,10 @@ extern void warn_slowpath_null(const char *file, const int line);
        static bool __section(.data.unlikely) __warned;         \
        int __ret_warn_once = !!(condition);                    \
                                                                \
-       if (unlikely(__ret_warn_once))                          \
-               if (WARN_ON(!__warned))                         \
-                       __warned = true;                        \
+       if (unlikely(__ret_warn_once && !__warned)) {           \
+               __warned = true;                                \
+               WARN_ON(1);                                     \
+       }                                                       \
        unlikely(__ret_warn_once);                              \
 })
 
@@ -120,9 +121,10 @@ extern void warn_slowpath_null(const char *file, const int line);
        static bool __section(.data.unlikely) __warned;         \
        int __ret_warn_once = !!(condition);                    \
                                                                \
-       if (unlikely(__ret_warn_once))                          \
-               if (WARN(!__warned, format))                    \
-                       __warned = true;                        \
+       if (unlikely(__ret_warn_once && !__warned)) {           \
+               __warned = true;                                \
+               WARN(1, format);                                \
+       }                                                       \
        unlikely(__ret_warn_once);                              \
 })
 
@@ -130,9 +132,10 @@ extern void warn_slowpath_null(const char *file, const int line);
        static bool __section(.data.unlikely) __warned;         \
        int __ret_warn_once = !!(condition);                    \
                                                                \
-       if (unlikely(__ret_warn_once))                          \
-               if (WARN_TAINT(!__warned, taint, format))       \
-                       __warned = true;                        \
+       if (unlikely(__ret_warn_once && !__warned)) {           \
+               __warned = true;                                \
+               WARN_TAINT(1, taint, format);                   \
+       }                                                       \
        unlikely(__ret_warn_once);                              \
 })