Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API

[Jann Horn]

On Fri, May 21, 2021 at 9:09 PM Andrei Vagin <avagin@xxxxxxxxx> wrote:
> On Thu, May 20, 2021 at 11:36:09AM -0700, Peter Oskolkov wrote:
> > @@ -67,7 +137,75 @@ SYSCALL_DEFINE4(umcg_register_task, u32, api_version, u32, flags, u32, group_id,
> >   */
> >  SYSCALL_DEFINE1(umcg_unregister_task, u32, flags)
> >  {
> > -     return -ENOSYS;
> > +     struct umcg_task_data *utd;
> > +     int ret = -EINVAL;
> > +
> > +     rcu_read_lock();
> > +     utd = rcu_dereference(current->umcg_task_data);
> > +
> > +     if (!utd || flags)
> > +             goto out;
> > +
> > +     task_lock(current);
> > +     rcu_assign_pointer(current->umcg_task_data, NULL);
> > +     task_unlock(current);
> > +
> > +     ret = 0;
> > +
> > +out:
> > +     rcu_read_unlock();
> > +     if (!ret && utd) {
> > +             synchronize_rcu();
>
> synchronize_rcu is expensive. Do we really need to call it here? Can we
> use kfree_rcu?
>
> Where is task->umcg_task_data freed when a task is destroyed?

or executed - the umcg stuff includes a userspace pointer, so it
probably shouldn't normally be kept around across execve?