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

Paul E. McKenney
2017-09-25T04:56:40Z

On Mon, Sep 25, 2017 at 06:41:30AM +0200, Steven Rostedt wrote:
> Sorry for the top post, currently on a train to Paris.
>
> This series already went through all my testing, and I would hate to rebase it for this reason. Can you just add a patch to remove the READ_ONCE()s?

If Linus accepts the original series, easy enough.

Thanx, Paul

> Thanks,
>
> -- Steve
>
>
> On September 25, 2017 2:34:56 AM GMT+02:00, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >On Sun, Sep 24, 2017 at 05:26:53PM -0700, Paul E. McKenney wrote:
> >> 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.
> >
> >And for whatever it is worth, here is the updated patch.
> >
> > Thanx, Paul
> >
> >------------------------------------------------------------------------
> >
> >commit 3e2baa988b9c13095995c46c51e0e32c0b6a7d43
> >Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >Date: Fri Sep 22 13:14:42 2017 -0700
> >
> > rcu: Allow for page faults in NMI handlers
> >
> > A number of architecture invoke rcu_irq_enter() on exception entry in
> >order to allow RCU read-side critical sections in the exception handler
> > when the exception is from an idle or nohz_full CPU. This works, at
> > least unless the exception happens in an NMI handler. In that case,
> >rcu_nmi_enter() would already have exited the extended quiescent state,
> > which would mean that rcu_irq_enter() would (incorrectly) cause RCU
> > to think that it is again in an extended quiescent state. This will
> > in turn result in lockdep splats in response to later RCU read-side
> > critical sections.
> >
> > This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to
> > take no action if there is an rcu_nmi_enter() in effect, thus avoiding
> > the unscheduled return to RCU quiescent state. This in turn should
> > make the kernel safe for on-demand RCU voyeurism.
> >
> > Reported-by: Steven Rostedt <rostedt@goodmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > [ paulmck: Remove READ_ONCE() per Linux Torvalds feedback. ]
> >
> >diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >index db5eb8c3f7af..e4fe06d42385 100644
> >--- a/kernel/rcu/tree.c
> >+++ b/kernel/rcu/tree.c
> >@@ -891,6 +891,11 @@ void rcu_irq_exit(void)
> >
> > RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs
> >enabled!!!");
> > rdtp = this_cpu_ptr(&rcu_dynticks);
> >+
> >+ /* Page faults can happen in NMI handlers, so check... */
> >+ if (rdtp->dynticks_nmi_nesting)
> >+ return;
> >+
> > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > rdtp->dynticks_nesting < 1);
> > if (rdtp->dynticks_nesting <= 1) {
> >@@ -1036,6 +1041,11 @@ void rcu_irq_enter(void)
> >
> > RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs
> >enabled!!!");
> > rdtp = this_cpu_ptr(&rcu_dynticks);
> >+
> >+ /* Page faults can happen in NMI handlers, so check... */
> >+ if (rdtp->dynticks_nmi_nesting)
> >+ return;
> >+
> > oldval = rdtp->dynticks_nesting;
> > rdtp->dynticks_nesting++;
> > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
>