> > 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
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