Re: FAILED: patch "[PATCH] rcu: Allow for page faults in NMI handlers" failed to apply to 4.9-stable tree

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Oct 02, 2017 at 03:33:17PM -0400, Steven Rostedt wrote:
> 
> Paul,
> 
> Can you backport this patch to 4.9 and 4.4 if it is required. The other
> patches I posted (and also failed to apply, but should be backported)
> depend on this patch, if it is an issue with those kernels.

No problem!

How do we determine whether or not it is required?

Do we need to ask one of Bruce or Nick?  ;-)

							Thanx, Paul

> -- Steve
> 
> 
> On Mon, 02 Oct 2017 13:44:37 +0200
> <gregkh@linuxfoundation.org> wrote:
> 
> > The patch below does not apply to the 4.9-stable tree.
> > If someone wants it applied there, or to any other stable or longterm
> > tree, then please email the backport, including the original git commit
> > id to <stable@vger.kernel.org>.
> > 
> > thanks,
> > 
> > greg k-h
> > 
> > ------------------ original commit in Linus's tree ------------------
> > 
> > >From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001  
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Date: Fri, 22 Sep 2017 14:10:22 -0700
> > Subject: [PATCH] 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.
> > 
> > Link: http://lkml.kernel.org/r/20170922211022.GA18084@linux.vnet.ibm.com
> > 
> > Cc: stable@vger.kernel.org
> > Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
> > Reported-by: Steven Rostedt <rostedt@goodmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 51d4c3acf32d..63bee8e1b193 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -888,6 +888,11 @@ 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 (READ_ONCE(rdtp->dynticks_nmi_nesting))
> > +		return;
> > +
> >  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> >  		     rdtp->dynticks_nesting < 1);
> >  	if (rdtp->dynticks_nesting <= 1) {
> > @@ -1020,6 +1025,11 @@ 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 (READ_ONCE(rdtp->dynticks_nmi_nesting))
> > +		return;
> > +
> >  	oldval = rdtp->dynticks_nesting;
> >  	rdtp->dynticks_nesting++;
> >  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
>