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 08:59:30AM +0100, David Hildenbrand wrote:
> > The compiler is within its rights to optimize the active_writer local
> > variable out of existence, thus re-introducing the possible race with
> > the writer that can pass a NULL pointer to wake_up_process().  So you
> > really need the ACCESS_ONCE() on the read from cpu_hotplug.active_writer.
> > Please see http://lwn.net/Articles/508991/ for more information why
> > this is absolutely required.
> 
> You're absolutely right, saw your reply on the other patch just after I sent
> this version ...
> 
> So if you agree with the change below, I'll send an updated version!
> 
> > 
> > > +		if (unlikely(active_writer))
> > > +			wake_up_process(active_writer);
> > >  		cpuhp_lock_release();
> > >  		return;
> > >  	}
> > > @@ -161,15 +167,17 @@ void cpu_hotplug_begin(void)
> > >  	cpuhp_lock_acquire();
> > >  	for (;;) {
> > >  		mutex_lock(&cpu_hotplug.lock);
> > > +		__set_current_state(TASK_UNINTERRUPTIBLE);
> > 
> > You lost me on this one.  How does this help?
> > 
> > 							Thanx, Paul
> 
> Imagine e.g. the following (simplified) scenario:
> 
> CPU1                               CPU2
> ----------------------------------------------------------------------------
> !mutex_trylock(&cpu_hotplug.lock) |
>                                   | cpu_hotplug.puts_pending == 0
> cpu_hotplug.puts_pending++;       |
>                                   | cpu_hotplug.refcount != 0
> wake_up_process(active_writer)
>                                   | __set_current_state(TASK_UNINTERRUPTIBLE);
>                                   | schedule();
>                                   | /* will never be woken up */
> 
> 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?