sched, signals: fix the racy usage of ->signal in account_group_xxx/run_posix_cpu_timers

Impact: fix potential NULL dereference

Contrary to ad474caca3e2a0550b7ce0706527ad5ab389a4d4 changelog, other
acct_group_xxx() helpers can be called after exit_notify() by timer tick.
Thanks to Roland for pointing out this. Somehow I missed this simple fact
when I read the original patch, and I am afraid I confused Frank during
the discussion. Sorry.

Fortunately, these helpers work with current, we can check ->exit_state
to ensure that ->signal can't go away under us.

Also, add the comment and compiler barrier to account_group_exec_runtime(),
to make sure we load ->signal only once.

Signed-off-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/posix-cpu-timers.c

···
  */
 static inline int fastpath_timer_check(struct task_struct *tsk)
 {
-	struct signal_struct *sig = tsk->signal;
+	struct signal_struct *sig;
 
-	if (unlikely(!sig))
+	/* tsk == current, ensure it is safe to use ->signal/sighand */
+	if (unlikely(tsk->exit_state))
 		return 0;
 
 	if (!task_cputime_zero(&tsk->cputime_expires)) {
···
 		if (task_cputime_expired(&task_sample, &tsk->cputime_expires))
 			return 1;
 	}
+
+	sig = tsk->signal;
 	if (!task_cputime_zero(&sig->cputime_expires)) {
 		struct task_cputime group_sample;
 

kernel/sched_stats.h

···
 {
 	struct signal_struct *sig;
 
-	sig = tsk->signal;
-	if (unlikely(!sig))
+	/* tsk == current, ensure it is safe to use ->signal */
+	if (unlikely(tsk->exit_state))
 		return;
+
+	sig = tsk->signal;
 	if (sig->cputime.totals) {
 		struct task_cputime *times;
 
···
 {
 	struct signal_struct *sig;
 
-	sig = tsk->signal;
-	if (unlikely(!sig))
+	/* tsk == current, ensure it is safe to use ->signal */
+	if (unlikely(tsk->exit_state))
 		return;
+
+	sig = tsk->signal;
 	if (sig->cputime.totals) {
 		struct task_cputime *times;
 
···
 	struct signal_struct *sig;
 
 	sig = tsk->signal;
+	/* see __exit_signal()->task_rq_unlock_wait() */
+	barrier();
 	if (unlikely(!sig))
 		return;
+
 	if (sig->cputime.totals) {
 		struct task_cputime *times;