tile: do_hardwall_trap: do not play with task->sighand

1. do_hardwall_trap() checks ->sighand != NULL and then takes ->siglock.

   This is unsafe even if the task can't run (I assume it is pinned to
   the same CPU), its parent can reap the task and set ->sighand = NULL
   right after this check. Even if the compiler dosn't read ->sighand
   twice and this memory can't to away __group_send_sig_info() is wrong
   after that. Use do_send_sig_info().

2. Send SIGILL to the thread, not to the whole process. Unless it has
   the handler or blocked this kills the whole thread-group as before.
   IIUC, different threads can be bound to different rect's.

3. Check PF_EXITING instead of ->sighand. A zombie thread can go away
   but its ->sighand can be !NULL.
Reported-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/kernel/hardwall.c

@@ -268,12 +268,10 @@ void __kprobes do_hardwall_trap(struct pt_regs* regs, int fault_num)
 	found_processes = 0;
 	list_for_each_entry(p, &rect->task_head, thread.hardwall_list) {
 		BUG_ON(p->thread.hardwall != rect);
-		if (p->sighand) {
+		if (!(p->flags & PF_EXITING)) {
 			found_processes = 1;
 			pr_notice("hardwall: killing %d\n", p->pid);
-			spin_lock(&p->sighand->siglock);
-			__group_send_sig_info(info.si_signo, &info, p);
-			spin_unlock(&p->sighand->siglock);
+			do_send_sig_info(info.si_signo, &info, p, false);
 		}
 	}
 	if (!found_processes)