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

->signal->tty is protected by ->siglock, no need to take the global tty_mutex.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/array.c

@@ -346,20 +346,13 @@ static int do_task_stat(struct task_struct *task, char * buffer, int whole)
 	sigemptyset(&sigcatch);
 	cutime = cstime = utime = stime = cputime_zero;
 
-	mutex_lock(&tty_mutex);
 	rcu_read_lock();
 	if (lock_task_sighand(task, &flags)) {
 		struct signal_struct *sig = task->signal;
-		struct tty_struct *tty = sig->tty;
 
-		if (tty) {
-			/*
-			 * sig->tty is not stable, but tty_mutex
-			 * protects us from release_dev(tty)
-			 */
-			barrier();
-			tty_pgrp = tty->pgrp;
-			tty_nr = new_encode_dev(tty_devnum(tty));
+		if (sig->tty) {
+			tty_pgrp = sig->tty->pgrp;
+			tty_nr = new_encode_dev(tty_devnum(sig->tty));
 		}
 
 		num_threads = atomic_read(&sig->count);
@@ -395,7 +388,6 @@ static int do_task_stat(struct task_struct *task, char * buffer, int whole)
 		unlock_task_sighand(task, &flags);
 	}
 	rcu_read_unlock();
-	mutex_unlock(&tty_mutex);
 
 	if (!whole || num_threads<2)
 		wchan = get_wchan(task);