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 03:47:48PM -0400, Steven Rostedt wrote:
> On Mon, 2 Oct 2017 12:38:57 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > 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?
> 
> Can rcu_irq_enter() be called within a rcu_nmi_enter() in these
> kernels? If not, that needs to be fixed. Because nmi's can still page
> fault.
> 
> > 
> > Do we need to ask one of Bruce or Nick?  ;-)
> > 
> 
> The bug I fixed is actually there, but lockdep doesn't complain. The
> module code that uses synchronize_sched() which requires RCU to be
> watching when preemption is disabled. RCU isn't watching is some of
> these cases that are called. Recent kernels added rcu annotations into
> these preempt disabled paths which caused lockdep to report the issue.
> The older kernels that had the bug just didn't have lockdep complain
> about it.

It turned out that one backport is needed for 4.2-4.4, and another
for 4.5-4.12.  I sent both.

							Thanx, Paul