lockdep: warn about lockdep disabling after kernel taint

Impact: provide useful missing info for developers

Kernel taint can occur in several situations such as warnings,
load of prorietary or staging modules, bad page, etc...

But when such taint happens, a developer might still be working on
the kernel, expecting that lockdep is still enabled. But a taint
disables lockdep without ever warning about it.
Such a kernel behaviour doesn't really help for kernel development.

This patch adds this missing warning.

Since the taint is done most of the time after the main message that
explain the real source issue, it seems safe to warn about it inside
add_taint() so that it appears at last, without hurting the main
information.

v2: Use a generic helper to disable lockdep instead of an
    open coded xchg().
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1239412638-6739-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/debug_locks.h

@@ -2,12 +2,19 @@
 #define __LINUX_DEBUG_LOCKING_H
 
 #include <linux/kernel.h>
+#include <asm/atomic.h>
 
 struct task_struct;
 
 extern int debug_locks;
 extern int debug_locks_silent;
 
+
+static inline int __debug_locks_off(void)
+{
+	return xchg(&debug_locks, 0);
+}
+
 /*
  * Generic 'turn off all lock debugging' function:
  */

kernel/panic.c

@@ -213,8 +213,14 @@ unsigned long get_taint(void)
 
 void add_taint(unsigned flag)
 {
-	/* can't trust the integrity of the kernel anymore: */
-	debug_locks = 0;
+	/*
+	 * Can't trust the integrity of the kernel anymore.
+	 * We don't call directly debug_locks_off() because the issue
+	 * is not necessarily serious enough to set oops_in_progress to 1
+	 */
+	if (__debug_locks_off())
+		printk(KERN_WARNING "Disabling lockdep due to kernel taint\n");
+
 	set_bit(flag, &tainted_mask);
 }
 EXPORT_SYMBOL(add_taint);

lib/debug_locks.c

@@ -36,7 +36,7 @@ int debug_locks_silent;
  */
 int debug_locks_off(void)
 {
-	if (xchg(&debug_locks, 0)) {
+	if (__debug_locks_off()) {
 		if (!debug_locks_silent) {
 			oops_in_progress = 1;
 			console_verbose();