quoted

especially while shutting down objects as they enter a lingering stage
where they're de-registered but not destroyed and you should be
careful which parts of the object are still accessible.  I haven't
read it carefully but here I'm not sure whether it's safe to do event
related operations after removal.  From cgroup core side, event list
is shut down synchronously from cgroup_destroy_locked().  It doesn't
seem like that part is explicitly built to remain accessible
afterwards.

/me goes and checks the code

vmpressure_event sends signals to _registered_ events but those are
unregistered from the work queue context by cgroup_event_remove (via
vmpressure_unregister_event) queued from cgroup_destroy_locked.

I am not sure what are the guarantees for ordering on the workqueue but
this all suggests that either vmpressure_event sees an empty vmpr->events
or it can safely send signals as cgroup_event_remove is pending on the
queue.

cgroup_event_remove drops a reference to cgrp->dentry after everything
is unregistered and event->wait removed from the wait queue so
cgroup_free_fn couldn't have been called yet and so memcg is still
alive. This means that even css_get/put is not necessary.

So I guess we are safe with the code as is but this all is really
_tricky_ and deserves a fat comment. So rather than adding flushing work
item code we should document it properly.

Or am I missing something?

But if I read the code correctly, even no one registers a vmpressure event,
vmpressure() is always running and queue the work item.