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Date:   Thu, 9 Nov 2017 09:56:54 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     Steven Rostedt <rostedt@...dmis.org>, stable@...r.kernel.org,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [4.4,06/28] rcu: Allow for page faults in NMI handlers

On Thu, Nov 09, 2017 at 03:17:28PM +0000, Ben Hutchings wrote:
> On Mon, 2017-10-16 at 18:11 +0200, gregkh@...uxfoundation.org wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > commit 28585a832602747cbfa88ad8934013177a3aae38 upstream.
> > 
> > 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@...r.kernel.org
> > Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
> > > Reported-by: Steven Rostedt <rostedt@...dmis.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > ---
> >  kernel/rcu/tree.c |   12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -759,6 +759,12 @@ void rcu_irq_exit(void)
> >  
> >  	local_irq_save(flags);
> >  	rdtp = this_cpu_ptr(&rcu_dynticks);
> > +
> > +	/* Page faults can happen in NMI handlers, so check... */
> > +	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
> > +		return;
> 
> Shouldn't there be a local_irq_restore() on this return path?  Or does
> this condition imply that IRQs were already disabled?
> 
> > +	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
> 
> I don't see why you added RCU_LOCKDEP_WARN() here.  Prior to 4.5 it's
> not an error to call this function with IRQs disabled.  And after
> calling local_irq_save(), it's redundant to assert that IRQs are
> disabled.
> 
> >  	oldval = rdtp->dynticks_nesting;
> >  	rdtp->dynticks_nesting--;
> >  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > @@ -887,6 +893,12 @@ void rcu_irq_enter(void)
> >  
> >  	local_irq_save(flags);
> >  	rdtp = this_cpu_ptr(&rcu_dynticks);
> > +
> > +	/* Page faults can happen in NMI handlers, so check... */
> > +	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
> > +		return;
> > +
> > +	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!");
> 
> Same problems here.

Indeed, it looks like I need to rework this for 4.5 and earlier.
Sorry for the noise!

							Thanx, Paul

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