Have you verified that the scenario you're describing can actually
happen?  AFAICS, cgroup_migrate_add_src() already does the pinning.

Hmmm. Looking at it, group_migrate_add_src() does grab a ref on the
css_set which contains the css, and the comments mention that grabbing
a ref on the css_set will stop the css from being dropped. I must've
misunderstood one of your previous emails (where you said that such
code was not safe in the ->can_fork(), ->cancel_fork() and ->fork())
path.

You also mentioned that depending on the css_set's ref being bumped to
protect the contained csses is "sort of implementation detail.  It can
be implemented in different ways." Which made me think that depending
on that is not a good idea.

But if it's safe to depend on the cgroup_migrate_add_src() pinning the
ref on the css_set, I'll drop this patch and fix up the other one
accordingly.

-- 
Aleksa Sarai (cyphar)
www.cyphar.com