Re: [PATCH] x86: Remove WARN_ON(in_nmi()) from vmalloc_fault

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

On Wed, Oct 16, 2013 at 09:37:12AM -0400, Steven Rostedt wrote:
> On Wed, 16 Oct 2013 15:28:15 +0200
> Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > On Wed, Oct 16, 2013 at 09:14:37AM -0400, Steven Rostedt wrote:
> > > On Wed, 16 Oct 2013 15:08:57 +0200
> > > Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > 
> > > 
> > > > Faults can call rcu_user_exit() / rcu_user_enter(). This is not supposed to happen
> > > > between rcu_nmi_enter() and rcu_nmi_exit(). rdtp->dynticks would be incremented in the
> > > > wrong way.
> > > > 
> > > > Ah but we have an in_interrupt() check in context_tracking_user_enter() that protects
> > > > us against that.
> > > 
> > > I will say that we should probably warn if it's any fault other than a
> > > vmalloc fault. A vmalloc fault should only happen in kernel space, and
> > > should not be happening from user code.
> > 
> > The NMI can interrupt userspace. When the fault happens, it sees that context tracking
> > state is set to userspace (NMIs and interrupts in general don't exit that state, hence
> > the in_interrupt() check that returns when user_exit/enter is called) so it calls user_enter().
> > But anyway we should be protected against that.
> 
> IIRC, NMI itself is safe to use rcu_read_lock(), at least I remember
> Paul making sure that stuff was lockless and NMI safe.

Yep, even preemptible RCU.  This relies on the fact that we cannot be
preempted within either an NMI handler or an exception handler.

> > > The WARN_ON() that I removed is from vmalloc fault. I don't see an
> > > issue with NMIs faulting via vmalloc. For any other page fault, sure, I
> > > would be concerned about it. But what's wrong with an NMI running
> > > module code?
> > 
> > I won't argue further as none of us is going to change his opinion on this :)
> 
> Sure sure, yet another argument continues with two sides stubbornly
> refusing to negotiate about a looming future (de)fault!

I figure some good hard testing will bring the truth of the matter to light.
The arguing parties might well then wish that they had compromised so as
to avoid the hard sharp truth, but by then it will be too late.  ;-)

							Thanx, Paul