Re: [PATCH 1/4] rcu: Allow for page faults in NMI handlers

Paul E. McKenney
2017-09-25T00:27:03Z

On Sun, Sep 24, 2017 at 05:12:13PM -0700, Linus Torvalds wrote:
> On Sun, Sep 24, 2017 at 5:03 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> >
> > Mostly just paranoia on my part. I would be happy to remove it if
> > you prefer. Or you or Steve can do so if that is more convenient.
>
> I really don't think it's warranted. The values are *stable*. There's
> no subtle lack of locking, or some optimistic access to a value that
> can change.
>
> The compiler can generate code to read the value fifteen billion
> times, and it will always get the same value.
>
> Yes, maybe in between the different accesses, an NMI will happen, and
> the value will be incremented, but then as the NMI exits, it will
> decrement again, so the code that got interrupted will not actually
> see the change.
>
> So the READ_ONCE() isn't "paranoia". It's just confusing.
>
> > And yes, consistency would dictate that the uses in rcu_nmi_enter()
> > and rcu_nmi_exit() should be _ONCE(), particularly the stores to
> > ->dynticks_nmi_nesting.
>
> NO.
>
> That would be just more of that confusion.
>
> That value is STABLE. It's stable even within an NMI handler. The NMI
> code can read it, modify it, write it back, do a little dance, all
> without having to care. There's no "_ONCE()" about it - not for the
> readers, not for the writers, not for _anybody_.
>
> So adding even more READ/WRITE_ONCE() accesses wouldn't be
> "consistent", it would just be insanity.
>
> Now, if an NMI happens and the value would be different on entry than
> it is on exit, that would be something else. Then it really wouldn't
> be stable wrt random users. But that would also be a major bug in the
> NMI handler, as far as I can tell.
>
> So the reason I'm objecting to that READ_ONCE() is that it isn't
> "paranoia", it's "voodoo programming". And we don't do voodoo
> programming.

I already agreed that the READ_ONCE() can be removed.

But without the WRITE_ONCE(), the compiler could theoretically tear
the store. Now we might be asserting that our compilers don't do that,
and that if they ever do, we will file a bug or whatever.

So are we asserting that our compilers won't ever do store tearing?

Thanx, Paul