Re: [PATCH -stable] rcu: Allow for page faults in NMI handlers

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

On Fri, Oct 13, 2017 at 06:33:52PM +0200, Greg KH wrote:
> On Fri, Oct 13, 2017 at 09:25:43AM -0700, Paul E. McKenney wrote:
> > On Fri, Oct 13, 2017 at 11:52:45AM +0200, Greg KH wrote:
> > > On Thu, Oct 12, 2017 at 04:09:28PM -0700, Paul E. McKenney wrote:
> > > > 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 # 4.2.x
> > > > 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>
> > > 
> > > This does not apply to the 4.4-stable tree, was this the correct patch I
> > > should be using for that tree?
> > 
> > Hmmm...  Idiot here backported to v4.4 in Linus's tree, which of
> > course might not work...  But it applies to v4.4.92 of -stable:
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
> > 
> > Am I still applying it in the wrong place?
> 
> Ugh, no, you are doing the right thing.  I don't know what failed on my
> end, that's what I get for trying to deal with RCU early in the morning
> :)

Believe me, I know that feeling very well!  ;-)

							Thanx, Paul

> I'll go apply this now, sorry for the noise, and thanks for checking and
> the backport.
> 
> greg k-h
>