linux-kernel - Re: On trace_*_rcuidle functions in modules

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Date:   Wed, 15 Apr 2020 17:49:18 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     John Stultz <john.stultz@...aro.org>
Cc:     paulmck@...nel.org, Josh Triplett <josh@...htriplett.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Todd Kjos <tkjos@...gle.com>, Stephen Boyd <sboyd@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: On trace_*_rcuidle functions in modules

On Wed, 15 Apr 2020 14:02:04 -0700
John Stultz <john.stultz@...aro.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?

-- Steve