[PATCH tip/core/rcu 04/17] rcu: Clamp ->dynticks_nmi_nesting at eqs entry/exit

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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  | 2 ++
 kernel/rcu/tree.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 59c471de342a..f4a411964c41 100644
--- a/kernel/rcu/rcu.h
+++ b/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.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 419f3c38e1b6..142cdd4a50c9 100644
--- a/kernel/rcu/tree.c
+++ b/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);
 	}
 }
 
-- 
2.5.2