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?
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