linux-kernel - Re: Reconciling rcu_irq_enter()/rcu_nmi

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Date:	Sat, 18 Jul 2015 15:00:04 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: Reconciling rcu_irq_enter()/rcu_nmi_enter() with context tracking

On Thu, Jul 16, 2015 at 06:53:15PM -0700, Andy Lutomirski wrote:
> For reasons that mystify me a bit, we currently track context tracking
> state separately from rcu's watching state.  This results in strange
> artifacts: nothing generic cause IRQs to enter CONTEXT_KERNEL, and we
> can nest exceptions inside the IRQ handler (an example would be
> wrmsr_safe failing), and, in -next, we splat a warning:
> 
> https://gist.github.com/sashalevin/a006a44989312f6835e7

I don't know how it happened. But the context tracking code should
be able to handle exceptions on irqs. They are supposed to be simply
ignored with the in_interrupt() check on context_tracking_enter/exit().

> 
> I'm trying to make context tracking more exact, which will fix this
> issue (the particular splat that Sasha hit shouldn't be possible when
> I'm done), but I think it would be nice to unify all of this stuff.
> Would it be plausible for us to guarantee that RCU state is always in
> sync with context tracking state?  If so, we could maybe simplify
> things and have fewer state variables.

RCU uses the same variables for idle and user tracking whereas context
tracking only tracks user. So they are at least decoupled there. And we
probably don't want RCU to use a different variable due to the overhead
it brings on readers. But it could be a shifted count on the same variable.

> 
> Doing this for NMIs might be weird.  Would it make sense to have a
> CONTEXT_NMI that's somehow valid even if the NMI happened while
> changing context tracking state.
> 
> Thoughts?  As it stands, I think we might already be broken for real:
> 
> Syscall -> user_exit.  Perf NMI hits *during* user_exit.  Perf does
> copy_from_user_nmi, which can fault, causing do_page_fault to get
> called, which calls exception_enter(), which can't be a good thing.

I think the in_interrupt() handles that. Besides NMI has its own counter.

> 
> RCU is okay (sort of) because of rcu_nmi_enter, but this seems very fragile.
> 
> Thoughts?  As it stands, I need to do something because -tip and thus
> -next spews occasional warnings.

But yeah if we can, it would be nice to use context tracking as the sole
tracker that RCU can safely use.
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