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

[Greg KH <gregkh@linuxfoundation.org>]

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?

thanks,

greg k-h