rcu: Clamp ->dynticks_nmi_nesting at eqs entry/exit

In preparation for merging dyntick-idle irq handling into the NMI
algorithm, clamp ->dynticks_nmi_nesting value to allow for interrupts
that enter but never leave and vice versa.

It is important that the clamping happen outside of the extended quiescent
state.  Otherwise, there will be short windows where irqs and NMIs fail
to convince RCU to start watching.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcu.h

@@ -56,6 +56,8 @@
 #define DYNTICK_TASK_EXIT_IDLE	   (DYNTICK_TASK_NEST_VALUE + \
 				    DYNTICK_TASK_FLAG)
 
+#define DYNTICK_IRQ_NONIDLE	((INT_MAX / 2) + 1)
+
 
 /*
  * Grace-period counter management.

kernel/rcu/tree.c

@@ -818,6 +818,7 @@ static void rcu_eqs_enter(bool user)
 	struct rcu_dynticks *rdtp;
 
 	rdtp = this_cpu_ptr(&rcu_dynticks);
+	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0);
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
 		     (rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK) == 0);
 	if ((rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK) == DYNTICK_TASK_NEST_VALUE)
@@ -1008,6 +1009,7 @@ static void rcu_eqs_exit(bool user)
 		rcu_eqs_exit_common(DYNTICK_TASK_EXIT_IDLE, user);
 		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
 		__this_cpu_dec(disable_rcu_irq_enter);
+		WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
 	}
 }