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

[Heiko Carstens <heiko.carstens@de.ibm.com>]

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?