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>

1 files changed, 12 insertions, 4 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 05d52185139c..557f3678c4e4 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -214,10 +214,18 @@ static void update_if_frozen(struct cgroup *cgroup,
 
         cgroup_iter_start(cgroup, &it);
         while ((task = cgroup_iter_next(cgroup, &it))) {
-                ntotal++;
-                if (freezing(task) && (frozen(task) ||
-                                       task_is_stopped_or_traced(task)))
-                        nfrozen++;
+                if (freezing(task)) {
+                        ntotal++;
+                        /*
+                         * 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) {