[PATCH] do_acct_process(): don't take tty_mutex

No need to take the global tty_mutex, signal->tty->driver can't go away while
we are holding ->siglock.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/acct.c

@@ -485,12 +485,9 @@ static void do_acct_process(struct file *file)
 	ac.ac_ppid = current->parent->tgid;
 #endif
 
-	mutex_lock(&tty_mutex);
-	tty = get_current_tty();
-	ac.ac_tty = tty ? old_encode_dev(tty_devnum(tty)) : 0;
-	mutex_unlock(&tty_mutex);
-
 	spin_lock_irq(&current->sighand->siglock);
+	tty = current->signal->tty;
+	ac.ac_tty = tty ? old_encode_dev(tty_devnum(tty)) : 0;
 	ac.ac_utime = encode_comp_t(jiffies_to_AHZ(cputime_to_jiffies(pacct->ac_utime)));
 	ac.ac_stime = encode_comp_t(jiffies_to_AHZ(cputime_to_jiffies(pacct->ac_stime)));
 	ac.ac_flag = pacct->ac_flag;