[PATCH] tty: fix the locking for signal->session in disassociate_ctty

commit 24ec839c while fixing the locking for
signal->tty got the locking wrong for signal->session.  This places our
accesses of signal->session back under the tasklist_lock where they belong.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_io.c

@@ -1496,7 +1496,6 @@ void disassociate_ctty(int on_exit)
 {
 	struct tty_struct *tty;
 	int tty_pgrp = -1;
-	int session;
 
 	lock_kernel();
 
@@ -1530,7 +1529,6 @@ void disassociate_ctty(int on_exit)
 
 	spin_lock_irq(&current->sighand->siglock);
 	current->signal->tty_old_pgrp = 0;
-	session = process_session(current);
 	spin_unlock_irq(&current->sighand->siglock);
 
 	mutex_lock(&tty_mutex);
@@ -1549,7 +1547,7 @@ void disassociate_ctty(int on_exit)
 
 	/* Now clear signal->tty under the lock */
 	read_lock(&tasklist_lock);
-	session_clear_tty(session);
+	session_clear_tty(process_session(current));
 	read_unlock(&tasklist_lock);
 	unlock_kernel();
 }