1 files changed, 49 insertions, 17 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7680fc275036..4c106fcc0d54 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -759,39 +759,71 @@ void rcu_irq_enter(void)
 /**
  * rcu_nmi_enter - inform RCU of entry to NMI context
  *
- * If the CPU was idle with dynamic ticks active, and there is no
- * irq handler running, this updates rdtp->dynticks_nmi to let the
- * RCU grace-period handling know that the CPU is active.
+ * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
+ * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
+ * that the CPU is active.  This implementation permits nested NMIs, as
+ * long as the nesting level does not overflow an int.  (You will probably
+ * run out of stack space first.)
  */
 void rcu_nmi_enter(void)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+	int incby = 2;
 
-	if (rdtp->dynticks_nmi_nesting == 0 &&
-	    (atomic_read(&rdtp->dynticks) & 0x1))
-		return;
-	rdtp->dynticks_nmi_nesting++;
-	smp_mb__before_atomic();  /* Force delay from prior write. */
-	atomic_inc(&rdtp->dynticks);
-	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
-	smp_mb__after_atomic();  /* See above. */
-	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
+	/* Complain about underflow. */
+	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting < 0);
+
+	/*
+	 * If idle from RCU viewpoint, atomically increment ->dynticks
+	 * to mark non-idle and increment ->dynticks_nmi_nesting by one.
+	 * Otherwise, increment ->dynticks_nmi_nesting by two.  This means
+	 * if ->dynticks_nmi_nesting is equal to one, we are guaranteed
+	 * to be in the outermost NMI handler that interrupted an RCU-idle
+	 * period (observation due to Andy Lutomirski).
+	 */
+	if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
+		smp_mb__before_atomic();  /* Force delay from prior write. */
+		atomic_inc(&rdtp->dynticks);
+		/* atomic_inc() before later RCU read-side crit sects */
+		smp_mb__after_atomic();  /* See above. */
+		WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
+		incby = 1;
+	}
+	rdtp->dynticks_nmi_nesting += incby;
+	barrier();
 }
 
 /**
  * rcu_nmi_exit - inform RCU of exit from NMI context
  *
- * If the CPU was idle with dynamic ticks active, and there is no
- * irq handler running, this updates rdtp->dynticks_nmi to let the
- * RCU grace-period handling know that the CPU is no longer active.
+ * If we are returning from the outermost NMI handler that interrupted an
+ * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
+ * to let the RCU grace-period handling know that the CPU is back to
+ * being RCU-idle.
  */
 void rcu_nmi_exit(void)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
 
-	if (rdtp->dynticks_nmi_nesting == 0 ||
-	    --rdtp->dynticks_nmi_nesting != 0)
+	/*
+	 * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
+	 * (We are exiting an NMI handler, so RCU better be paying attention
+	 * to us!)
+	 */
+	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting <= 0);
+	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
+
+	/*
+	 * If the nesting level is not 1, the CPU wasn't RCU-idle, so
+	 * leave it in non-RCU-idle state.
+	 */
+	if (rdtp->dynticks_nmi_nesting != 1) {
+		rdtp->dynticks_nmi_nesting -= 2;
 		return;
+	}
+
+	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
+	rdtp->dynticks_nmi_nesting = 0;
 	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
 	smp_mb__before_atomic();  /* See above. */
 	atomic_inc(&rdtp->dynticks);