do_task_stat: don't take rcu_read_lock()

lock_task_sighand() was changed, and do_task_stat() doesn't need
rcu_read_lock any longer.  sighand->siglock protects all "interesting"
fields.

Except: it doesn't protect ->tty->pgrp, but neither does rcu_read_lock(), this
should be fixed.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Pavel Emelyanov <xemul@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/array.c

@@ -425,7 +425,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	cutime = cstime = utime = stime = cputime_zero;
 	cgtime = gtime = cputime_zero;
 
-	rcu_read_lock();
 	if (lock_task_sighand(task, &flags)) {
 		struct signal_struct *sig = task->signal;
 
@@ -469,7 +468,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 
 		unlock_task_sighand(task, &flags);
 	}
-	rcu_read_unlock();
 
 	if (!whole || num_threads < 2)
 		wchan = get_wchan(task);