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

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> > 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);
+         /* inc after get_task_struct(), so the writer can't get NULL */
                atomic_inc(&cpu_hotplug.puts_pending);
+         /* we might be the last one */
+         if (unlikely(awr)) {
+                 wake_up_process(awr);
+                 put_task_struct(awr);
+         }
                cpuhp_lock_release();
                return;
        }


Thanks!

David