Patch "rcu: Allow for page faults in NMI handlers" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    rcu: Allow for page faults in NMI handlers

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     rcu-allow-for-page-faults-in-nmi-handlers.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 28585a832602747cbfa88ad8934013177a3aae38 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date: Fri, 22 Sep 2017 14:10:22 -0700
Subject: rcu: Allow for page faults in NMI handlers

From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

commit 28585a832602747cbfa88ad8934013177a3aae38 upstream.

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
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/rcu/tree.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -759,6 +759,12 @@ void rcu_irq_exit(void)
 
 	local_irq_save(flags);
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/* Page faults can happen in NMI handlers, so check... */
+	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+		return;
+
+	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_exit() invoked with irqs enabled!!!");
 	oldval = rdtp->dynticks_nesting;
 	rdtp->dynticks_nesting--;
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
@@ -887,6 +893,12 @@ void rcu_irq_enter(void)
 
 	local_irq_save(flags);
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/* Page faults can happen in NMI handlers, so check... */
+	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+		return;
+
+	RCU_LOCKDEP_WARN(!irqs_disabled(), "rcu_irq_enter() invoked with irqs enabled!!!");
 	oldval = rdtp->dynticks_nesting;
 	rdtp->dynticks_nesting++;
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&


Patches currently in stable-queue which might be from paulmck@linux.vnet.ibm.com are

queue-4.4/rcu-allow-for-page-faults-in-nmi-handlers.patch