tracing/filter: Use a tree instead of stack for filter_match_preds()

Currently the filter_match_preds() requires a stack to push
and pop the preds to determine if the filter matches the record or not.
This has two drawbacks:

1) It requires a stack to store state information. As this is done
   in fast paths we can't allocate the storage for this stack, and
   we can't use a global as it must be re-entrant. The stack is stored
   on the kernel stack and this greatly limits how many preds we
   may allow.

2) All conditions are calculated even when a short circuit exists.
   a || b  will always calculate a and b even though a was determined
   to be true.

Using a tree we can walk a constant structure that will save
the state as we go. The algorithm is simply:

  pred = root;
  do {
	switch (move) {
	case MOVE_DOWN:
		if (OR or AND) {
			pred = left;
			continue;
		}
		if (pred == root)
			break;
		match = pred->fn();
		pred = pred->parent;
		move = left child ? MOVE_UP_FROM_LEFT : MOVE_UP_FROM_RIGHT;
		continue;

	case MOVE_UP_FROM_LEFT:
		/* Only OR or AND can be a parent */
		if (match && OR || !match && AND) {
			/* short circuit */
			if (pred == root)
				break;
			pred = pred->parent;
			move = left child ?
				MOVE_UP_FROM_LEFT :
				MOVE_UP_FROM_RIGHT;
			continue;
		}
		pred = pred->right;
		move = MOVE_DOWN;
		continue;

	case MOVE_UP_FROM_RIGHT:
		if (pred == root)
			break;
		pred = pred->parent;
		move = left child ? MOVE_UP_FROM_LEFT : MOVE_UP_FROM_RIGHT;
		continue;
	}
	done = 1;
  } while (!done);

This way there's no strict limit to how many preds we allow
and it also will short circuit the logical operations when possible.

Cc: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -664,6 +664,7 @@ struct event_filter {
 	int			n_preds;	/* Number assigned */
 	int			a_preds;	/* allocated */
 	struct filter_pred	*preds;
+	struct filter_pred	*root;
 	char			*filter_string;
 };
 
@@ -675,6 +676,9 @@ struct event_subsystem {
 	int			nr_events;
 };
 
+#define FILTER_PRED_INVALID	((unsigned short)-1)
+#define FILTER_PRED_IS_RIGHT	(1 << 15)
+
 struct filter_pred;
 struct regex;
 
@@ -704,7 +708,10 @@ struct filter_pred {
 	int 			offset;
 	int 			not;
 	int 			op;
-	int 			pop_n;
+	unsigned short		index;
+	unsigned short		parent;
+	unsigned short		left;
+	unsigned short		right;
 };
 
 extern struct list_head ftrace_common_fields;