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

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Date:   Sun, 24 Sep 2017 17:26:53 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/4] rcu: Allow for page faults in NMI handlers

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@...ux.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