rcu: Disable preemption in rcu_is_cpu_idle()

Because rcu_is_cpu_idle() is to be used to check for extended quiescent
states in RCU-preempt read-side critical sections, it cannot assume that
preemption is disabled.  And preemption must be disabled when accessing
the dyntick-idle state, because otherwise the following sequence of events
could occur:

1.	Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer
	to CPU 1's per-CPU variables.

2.	Task B preempts Task A and starts running on CPU 1.

3.	Task A migrates to CPU 2.

4.	Task B blocks, leaving CPU 1 idle.

5.	Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle
	information using the pointer fetched in step 1 above, and finds
	that CPU 1 is idle.

6.	Task A therefore incorrectly concludes that it is executing in
	an extended quiescent state, possibly issuing a spurious splat.

Therefore, this commit disables preemption within the rcu_is_cpu_idle()
function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcutree.c

@@ -556,12 +556,16 @@ void rcu_nmi_exit(void)
  * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
  *
  * If the current CPU is in its idle loop and is neither in an interrupt
- * or NMI handler, return true.  The caller must have at least disabled
- * preemption.
+ * or NMI handler, return true.
  */
 int rcu_is_cpu_idle(void)
 {
-	return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
+	int ret;
+
+	preempt_disable();
+	ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
+	preempt_enable();
+	return ret;
 }
 
 #endif /* #ifdef CONFIG_PROVE_RCU */