Re: [PATCH v2] CPU hotplug: active_writer not woken up in some cases - deadlock

[David Hildenbrand]

> On Tue, Dec 09, 2014 at 11:11:01AM +0100, David Hildenbrand wrote:
> > > > Therefore we have to move the condition check inside the 
> > > >   __set_current_state(TASK_UNINTERRUPTIBLE) -> schedule();
> > > > section to not miss any wake ups when the condition is satisfied.
> > > > 
> > > > So wake_up_process() will either see TASK_RUNNING and do nothing or see
> > > > TASK_UNINTERRUPTIBLE and set it to TASK_RUNNING, so schedule() will in
> > > > fact be woken up again.
> > > 
> > > Or the third alternative would be that 'active_writer' which was running
> > > on CPU2 already terminated and wake_up_process() has a non-NULL pointer to
> > > task_struct which is already dead.
> > > Or is there anything that prevents this use-after-free race?
> > 
> > Hmmm ... I think that is also a valid scenario.
> > That would mean we need soemthing like this:
> > 
> >  void put_online_cpus(void)
> >  {
> > + struct task_struct *awr;
> > +
> >         if (cpu_hotplug.active_writer == current)
> >                 return;
> >         if (!mutex_trylock(&cpu_hotplug.lock)) {
> > +         awr = ACCESS_ONCE(cpu_hotplug.active_writer);
> > +         if (unlikely(awr))
> > +                 get_task_struct(awr);
> 
> How would this solve the problem?

If I am not completely wrong, an active_writer will remain in
it's loop (cpu_hotplug_begin) until the refcount is down to 0. As we are
putting the cpus, the refcount is > 0 (because of the previous get_all_cpus()
which incremented the refcount).

cpu_hotplug_begin will only be able to exit as soon as refcount == 0, therefore
in our special case if cpu_hotplug.puts_pending has been incremented.

As long as we don't increment cpu_hotplug.puts_pending, the active_writer will
not vanish. Therefore awr still points to a valid task struct after we
incremented cpu_hotplug.puts_pending.

get_task_struct() will make sure that the struct will not vanish after we
incremented cpu_hotplug.puts_pending (and therefore decremented the refcount).


Or am I missing something?

Thanks!

David

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