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

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Date:   Fri, 21 May 2021 23:31:52 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Peter Oskolkov <posk@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...k.io>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrei Vagin <avagin@...gle.com>,
        Jim Newsome <jnewsome@...project.org>
Subject: Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API

On Fri, May 21, 2021 at 9:09 PM Andrei Vagin <avagin@...il.com> 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?