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From:   Peter Oskolkov <posk@google.com>
Date:   Fri, 21 May 2021 15:03:18 -0700
Message-ID: <CAPNVh5fOPn+yuH5kEcGBFLphxDHSvsVS1QeFtHh63d4Go9UAJA@mail.gmail.com>
Subject: Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API
To:     Jann Horn <jannh@google.com>
Cc:     Andrei Vagin <avagin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Peter Oskolkov <posk@posk.io>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrei Vagin <avagin@google.com>,
        Jim Newsome <jnewsome@torproject.org>
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On Fri, May 21, 2021 at 2:32 PM Jann Horn <jannh@google.com> wrote:
>
> On Fri, May 21, 2021 at 9:09 PM Andrei Vagin <avagin@gmail.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?

Ack - thanks for these and other comments. Please keep them coming.
I'll address them in v0.2.

Thanks,
Peter