[PATCH] vt: rework the console spawning variables

This is such a rare path it took me a while to figure out how to test
this after soring out the locking.

This patch does several things.
- The variables used are moved into a structure and declared in vt_kern.h
- A spinlock is added so we don't have SMP races updating the values.
- Instead of raw pid_t value a struct_pid is used to guard against
  pid wrap around issues, if the daemon to spawn a new console dies.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/vt_ioctl.c

@@ -645,13 +645,16 @@ int vt_ioctl(struct tty_struct *tty, struct file * file,
 	 */
 	case KDSIGACCEPT:
 	{
-		extern int spawnpid, spawnsig;
 		if (!perm || !capable(CAP_KILL))
 		  return -EPERM;
 		if (!valid_signal(arg) || arg < 1 || arg == SIGKILL)
 		  return -EINVAL;
-		spawnpid = current->pid;
-		spawnsig = arg;
+
+		spin_lock_irq(&vt_spawn_con.lock);
+		put_pid(vt_spawn_con.pid);
+		vt_spawn_con.pid = get_pid(task_pid(current));
+		vt_spawn_con.sig = arg;
+		spin_unlock_irq(&vt_spawn_con.lock);
 		return 0;
 	}