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. -- Steve On Mon, 02 Oct 2017 13:44:37 +0200 <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > The patch below does not apply to the 4.9-stable tree. > If someone wants it applied there, or to any other stable or longterm > tree, then please email the backport, including the original git commit > id to <stable@xxxxxxxxxxxxxxx>. > > thanks, > > greg k-h > > ------------------ original commit in Linus's tree ------------------ > > >From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001 > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > Date: Fri, 22 Sep 2017 14:10:22 -0700 > Subject: [PATCH] rcu: Allow for page faults in NMI handlers > > 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@xxxxxxxxxxxxxxxxxx > > Cc: stable@xxxxxxxxxxxxxxx > Fixes: 0be964be0 ("module: Sanitize RCU usage and locking") > Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 51d4c3acf32d..63bee8e1b193 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -888,6 +888,11 @@ void rcu_irq_exit(void) > > RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!"); > rdtp = this_cpu_ptr(&rcu_dynticks); > + > + /* Page faults can happen in NMI handlers, so check... */ > + if (READ_ONCE(rdtp->dynticks_nmi_nesting)) > + return; > + > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && > rdtp->dynticks_nesting < 1); > if (rdtp->dynticks_nesting <= 1) { > @@ -1020,6 +1025,11 @@ void rcu_irq_enter(void) > > RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!"); > rdtp = this_cpu_ptr(&rcu_dynticks); > + > + /* Page faults can happen in NMI handlers, so check... */ > + if (READ_ONCE(rdtp->dynticks_nmi_nesting)) > + return; > + > oldval = rdtp->dynticks_nesting; > rdtp->dynticks_nesting++; > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&