TTY: tty_buffer, warn on leaks

When we leak some tty buffer, warn about that. For that we need to
account the memory used also in the tty_buffer_free_all function. On
other locations, the accounting is handled correctly.

Note that we do not account the free list, as that was accounted in
tty_buffer_free before put on the free list.

I have been using this patch for ages, so let's see if anybody else
encounters any issues.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -118,9 +118,12 @@ void tty_buffer_free_all(struct tty_port *port)
 	struct tty_bufhead *buf = &port->buf;
 	struct tty_buffer *p, *next;
 	struct llist_node *llist;
+	unsigned int freed = 0;
+	int still_used;
 
 	while ((p = buf->head) != NULL) {
 		buf->head = p->next;
+		freed += p->size;
 		if (p->size > 0)
 			kfree(p);
 	}
@@ -132,7 +135,9 @@ void tty_buffer_free_all(struct tty_port *port)
 	buf->head = &buf->sentinel;
 	buf->tail = &buf->sentinel;
 
-	atomic_set(&buf->mem_used, 0);
+	still_used = atomic_xchg(&buf->mem_used, 0);
+	WARN(still_used != freed, "we still have not freed %d bytes!",
+			still_used - freed);
 }
 
 /**