lockdep: Print a nicer description for normal deadlocks

The lockdep output can be pretty cryptic, having nicer output
can save a lot of head scratching. When a normal deadlock
scenario is detected by lockdep (lock A -> lock B and there
exists a place where lock B -> lock A) we now get the following
new output:

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockB);
                               lock(lockA);
                               lock(lockB);
  lock(lockA);

 *** DEADLOCK ***

On cases where there's a deeper chair, it shows the partial
chain that can cause the issue:

Chain exists of:
  lockC --> lockA --> lockB

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(lockB);
                               lock(lockA);
                               lock(lockB);
  lock(lockC);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.380621789@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/lockdep.c

@@ -1065,6 +1065,56 @@ print_circular_bug_entry(struct lock_list *target, int depth)
 	return 0;
 }
 
+static void
+print_circular_lock_scenario(struct held_lock *src,
+			     struct held_lock *tgt,
+			     struct lock_list *prt)
+{
+	struct lock_class *source = hlock_class(src);
+	struct lock_class *target = hlock_class(tgt);
+	struct lock_class *parent = prt->class;
+
+	/*
+	 * A direct locking problem where unsafe_class lock is taken
+	 * directly by safe_class lock, then all we need to show
+	 * is the deadlock scenario, as it is obvious that the
+	 * unsafe lock is taken under the safe lock.
+	 *
+	 * But if there is a chain instead, where the safe lock takes
+	 * an intermediate lock (middle_class) where this lock is
+	 * not the same as the safe lock, then the lock chain is
+	 * used to describe the problem. Otherwise we would need
+	 * to show a different CPU case for each link in the chain
+	 * from the safe_class lock to the unsafe_class lock.
+	 */
+	if (parent != source) {
+		printk("Chain exists of:\n  ");
+		__print_lock_name(source);
+		printk(" --> ");
+		__print_lock_name(parent);
+		printk(" --> ");
+		__print_lock_name(target);
+		printk("\n\n");
+	}
+
+	printk(" Possible unsafe locking scenario:\n\n");
+	printk("       CPU0                    CPU1\n");
+	printk("       ----                    ----\n");
+	printk("  lock(");
+	__print_lock_name(target);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(parent);
+	printk(");\n");
+	printk("                               lock(");
+	__print_lock_name(target);
+	printk(");\n");
+	printk("  lock(");
+	__print_lock_name(source);
+	printk(");\n");
+	printk("\n *** DEADLOCK ***\n\n");
+}
+
 /*
  * When a circular dependency is detected, print the
  * header first:
@@ -1108,6 +1158,7 @@ static noinline int print_circular_bug(struct lock_list *this,
 {
 	struct task_struct *curr = current;
 	struct lock_list *parent;
+	struct lock_list *first_parent;
 	int depth;
 
 	if (!debug_locks_off_graph_unlock() || debug_locks_silent)
@@ -1121,6 +1172,7 @@ static noinline int print_circular_bug(struct lock_list *this,
 	print_circular_bug_header(target, depth, check_src, check_tgt);
 
 	parent = get_lock_parent(target);
+	first_parent = parent;
 
 	while (parent) {
 		print_circular_bug_entry(parent, --depth);
@@ -1128,6 +1180,9 @@ static noinline int print_circular_bug(struct lock_list *this,
 	}
 
 	printk("\nother info that might help us debug this:\n\n");
+	print_circular_lock_scenario(check_src, check_tgt,
+				     first_parent);
+
 	lockdep_print_held_locks(curr);
 
 	printk("\nstack backtrace:\n");