[PATCH] uml: fix exitcall ordering bug

This fixes an exitcall ordering bug - calls to ignore_sigio_fd can come from
exitcalls that come after the sigio thread has been killed.  This would cause
shutdown to hang or crash.

Fixed by having ignore_sigio_fd check that the thread is present before trying
to communicate with it.
Signed-off-by: NJeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/um/os-Linux/sigio.c

@@ -191,6 +191,13 @@ int ignore_sigio_fd(int fd)
 	struct pollfd *p;
 	int err = 0, i, n = 0;
 
+	/* This is called from exitcalls elsewhere in UML - if
+	 * sigio_cleanup has already run, then update_thread will hang
+	 * or fail because the thread is no longer running.
+	 */
+	if(write_sigio_pid == -1)
+		return -EIO;
+
 	sigio_lock();
 	for(i = 0; i < current_poll.used; i++){
 		if(current_poll.poll[i].fd == fd) break;
@@ -215,7 +222,7 @@ int ignore_sigio_fd(int fd)
 	update_thread();
  out:
 	sigio_unlock();
-	return(err);
+	return err;
 }
 
 static struct pollfd *setup_initial_poll(int fd)