Re: On trace_*_rcuidle functions in modules

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Apr 15, 2020 at 05:49:18PM -0400, Steven Rostedt wrote:
> On Wed, 15 Apr 2020 14:02:04 -0700
> John Stultz <john.stultz@linaro.org> wrote:
> 
> > 
> > So in my case your concerns may not be a problem, but I guess
> > generally it might. Though I'd hope the callback would be unregistered
> > (and whatever waiting for the grace period to complete be done) before
> > the module removal is complete. But maybe I'm still missing your
> > point?
> 
> Hmm, you may have just brought up a problem here...
> 
> You're saying that cpu_pm_register_notifier() callers are called from non
> RCU watching context? If that's the case, we have this:
> 
> int cpu_pm_unregister_notifier(struct notifier_block *nb)
> {
> 	return atomic_notifier_chain_unregister(&cpu_pm_notifier_chain, nb);
> }
> 
> And this:
> 
> int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
> 		struct notifier_block *n)
> {
> 	unsigned long flags;
> 	int ret;
> 
> 	spin_lock_irqsave(&nh->lock, flags);
> 	ret = notifier_chain_unregister(&nh->head, n);
> 	spin_unlock_irqrestore(&nh->lock, flags);
> 	synchronize_rcu();
> 	return ret;
> }
> 
> Which means that if something registered a cpu_pm notifier, then
> unregistered it, and freed whatever the notifier accesses, then there's a
> chance that the synchronize_rcu() can return before the called notifier
> finishes, and anything that notifier accesses could have been freed.
> 
> I believe that module code should not be able to be run in RCU non watching
> context, and neither should notifiers. I think we just stumbled on a bug.
> 
> Paul?

Or we say that such modules cannot be unloaded.  Or that such modules'
exit handlers, after disentangling themselves from the idle loop, must
invoke synchronize_rcu_rude() or similar, just as modules that use
call_rcu() are currently required to invoke rcu_barrier().

Or is it possible to upgrade the protection that modules use?

My guess is that invoking rcu_irq_enter() and rcu_irq_exit() around every
potential call into module code out of the PM code is a non-starter,
but I cannot prove that either way.

							Thanx, Paul