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

[Steven Rostedt]

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?

Thanks,

-- Steve


On September 25, 2017 2:34:56 AM GMT+02:00, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
>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@xxxxxxxxxxx>
>    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>    [ 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) &&

-- 
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