[PATCH] disassociate_ctty SMP fix

Patch from Rik van Riel <riel@conectiva.com.br>

the following patch, against today's BK tree, fixes a small
SMP race in disassociate_ctty.  This function gets called
from do_exit, without the BKL held.

However, it sets the *tty variable before grabbing the bkl,
then makes decisions on what the variable was set to before
the lock was grabbed, despite the fact that another process
could modify its ->tty pointer in this same function.

drivers/char/tty_io.c

@@ -578,7 +578,7 @@ EXPORT_SYMBOL(tty_hung_up_p);
  */
 void disassociate_ctty(int on_exit)
 {
-	struct tty_struct *tty = current->tty;
+	struct tty_struct *tty;
 	struct task_struct *p;
 	struct list_head *l;
 	struct pid *pid;
@@ -586,6 +586,7 @@ void disassociate_ctty(int on_exit)
 
 	lock_kernel();
 
+	tty = current->tty;
 	if (tty) {
 		tty_pgrp = tty->pgrp;
 		if (on_exit && tty->driver.type != TTY_DRIVER_TYPE_PTY)