Date:   Sun, 24 Sep 2017 21:56:32 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.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 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@...ux.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@...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.
> >
> >And for whatever it is worth, here is the updated patch.
> >
> >							Thanx, Paul
> >
> >------------------------------------------------------------------------
> >
> >commit 3e2baa988b9c13095995c46c51e0e32c0b6a7d43
> >Author: Paul E. McKenney <paulmck@...ux.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@...dmis.org>
> >    Signed-off-by: Paul E. McKenney <paulmck@...ux.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.
> 

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