rcu: Remove extraneous READ_ONCE()s from rcu_irq_{enter,exit}()

The read of ->dynticks_nmi_nesting in rcu_irq_enter() and rcu_irq_exit()
is currently protected with READ_ONCE().  However, this protection is
unnecessary because (1) ->dynticks_nmi_nesting is updated only by the
current CPU, (2) Although NMI handlers can update this field, they reset
it back to its old value before return, and (3) Interrupts are disabled,
so nothing else can modify it.  The value of ->dynticks_nmi_nesting is
thus effectively constant, and so no protection is required.

This commit therefore removes the READ_ONCE() protection from these
two accesses.

Link: http://lkml.kernel.org/r/20170926031902.GA2074@linux.vnet.ibm.com

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/rcu/tree.c

@@ -890,7 +890,7 @@ void rcu_irq_exit(void)
 	rdtp = this_cpu_ptr(&rcu_dynticks);
 
 	/* Page faults can happen in NMI handlers, so check... */
-	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+	if (rdtp->dynticks_nmi_nesting)
 		return;
 
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
@@ -1027,7 +1027,7 @@ void rcu_irq_enter(void)
 	rdtp = this_cpu_ptr(&rcu_dynticks);
 
 	/* Page faults can happen in NMI handlers, so check... */
-	if (READ_ONCE(rdtp->dynticks_nmi_nesting))
+	if (rdtp->dynticks_nmi_nesting)
 		return;
 
 	oldval = rdtp->dynticks_nesting;