cgroup_freezer: don't stall transition to FROZEN for PF_NOFREEZE or PF_FREEZER_SKIP tasks

cgroup_freezer doesn't transition from FREEZING to FROZEN if the
cgroup contains PF_NOFREEZE tasks or tasks sleeping with
PF_FREEZER_SKIP set.

Only kernel tasks can be non-freezable (PF_NOFREEZE) and there's
nothing cgroup_freezer or userland can do about or to it.  It's
pointless to stall the transition for PF_NOFREEZE tasks.

PF_FREEZER_SKIP indicates that the task can be skipped when
determining whether frozen state is reached.  A task with
PF_FREEZER_SKIP is guaranteed to perform try_to_freeze() after it
wakes up and can be considered frozen much like stopped or traced
tasks.  Note that a vfork parent uses PF_FREEZER_SKIP while waiting
for the child.

This updates update_if_frozen() such that it only considers freezable
tasks and treats %true freezer_should_skip() tasks as frozen.

This allows cgroups w/ kthreads and vfork parents successfully reach
FROZEN state.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>

kernel/cgroup_freezer.c

@@ -214,11 +214,19 @@ static void update_if_frozen(struct cgroup *cgroup,
 
 	cgroup_iter_start(cgroup, &it);
 	while ((task = cgroup_iter_next(cgroup, &it))) {
+		if (freezing(task)) {
 			ntotal++;
-		if (freezing(task) && (frozen(task) ||
-				       task_is_stopped_or_traced(task)))
+			/*
+			 * freezer_should_skip() indicates that the task
+			 * should be skipped when determining freezing
+			 * completion.  Consider it frozen in addition to
+			 * the usual frozen condition.
+			 */
+			if (frozen(task) || task_is_stopped_or_traced(task) ||
+			    freezer_should_skip(task))
 				nfrozen++;
 		}
+	}
 
 	if (old_state == CGROUP_THAWED) {
 		BUG_ON(nfrozen > 0);