rcu: Integrate rcu_dereference_check() message into lockdep

Make rcu_dereference_check() print the list of held locks in
addition to the stack dump to ease debugging.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1266887105-1528-3-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/lockdep.h

@@ -534,4 +534,8 @@ do {									\
 # define might_lock_read(lock) do { } while (0)
 #endif
 
+#ifdef CONFIG_PROVE_RCU
+extern void lockdep_rcu_dereference(const char *file, const int line);
+#endif
+
 #endif /* __LINUX_LOCKDEP_H */

include/linux/rcupdate.h

@@ -182,8 +182,8 @@ static inline int rcu_read_lock_sched_held(void)
  */
 #define rcu_dereference_check(p, c) \
 	({ \
-		if (debug_locks) \
-			WARN_ON_ONCE(!(c)); \
+		if (debug_locks && !(c)) \
+			lockdep_rcu_dereference(__FILE__, __LINE__); \
 		rcu_dereference_raw(p); \
 	})
 

kernel/lockdep.c

@@ -3809,3 +3809,21 @@ void lockdep_sys_exit(void)
 		lockdep_print_held_locks(curr);
 	}
 }
+
+void lockdep_rcu_dereference(const char *file, const int line)
+{
+	struct task_struct *curr = current;
+
+	if (!debug_locks_off())
+		return;
+	printk("\n==============================================\n");
+	printk(  "[ BUG: Unsafe rcu_dereference_check() usage! ]\n");
+	printk(  "----------------------------------------------\n");
+	printk("%s:%d invoked rcu_dereference_check() without protection!\n",
+			file, line);
+	printk("\nother info that might help us debug this:\n\n");
+	lockdep_print_held_locks(curr);
+	printk("\nstack backtrace:\n");
+	dump_stack();
+}
+EXPORT_SYMBOL_GPL(lockdep_rcu_dereference);