Re: [PATCH v3] KVM: Move VM's worker kthreads back to the original cgroup before exiting.

Sean Christopherson
2022-02-19T00:31:03Z

On Thu, Feb 17, 2022, Paolo Bonzini wrote:
> On 2/17/22 13:34, kernel test robot wrote:
> > > 5859 reattach_err = cgroup_attach_task_all(current->real_parent, current);
>
> This needs to be within rcu_dereference().

As Vipin found out the hard way, cgroup_attach_task_all() can't be inside an RCU
critical section as it might sleep due to mutex_lock().

My sched knowledge is sketchy at best, but I believe KVM just needs to guarantee
the liveliness of real_parent when passing it to cgroup_attach_task_all().
Presumably kthreadd_task can (in theory) be killed/exiting while this is going on,
but at that point I doubt anyone cares much about cgroup accuracy.

So I think this can be:

rcu_read_lock();
parent = rcu_dereference(current->real_parent);
get_task_struct(parent);
rcu_read_unlock();

(void)cgroup_attach_task_all(parent, current);
put_task_struct(parent);

> > 5860 if (reattach_err) {
> > 5861 kvm_err("%s: cgroup_attach_task_all failed on reattach with err %d\n",
> > 5862 __func__, reattach_err);

Eh, I wouldn't bother logging the error, userspace can't take action on it, and
if the system is OOM it's just more noise in the log to dig through.