[PATCH RFC tip/core/rcu 1/6] rcu: Stabilize use of num_online_cpus() for GP short circuit

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

From: "Paul E. McKenney" <paul.mckenney@linaro.org>

The rcu_blocking_is_gp() function tests to see if there is only one
online CPU, and if so, synchronize_sched() and friends become no-ops.
However, for larger systems, num_online_cpus() scans a large vector,
and might be preempted while doing so.  While preempted, any number
of CPUs might come online and go offline, potentially resulting in
num_online_cpus() returning 1 when there never had only been one
CPU online.  This would result in a too-short RCU grace period, which
could in turn result in total failure.

This commit therefore protects the num_online_cpus() with preempt_disable().

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcutree.h |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index e8ee5dd..997179f 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -101,8 +101,13 @@ extern void rcu_sched_force_quiescent_state(void);
 /* A context switch is a grace period for RCU-sched and RCU-bh. */
 static inline int rcu_blocking_is_gp(void)
 {
+	int ret;
+
 	might_sleep();  /* Check for RCU read-side critical section. */
-	return num_online_cpus() == 1;
+	preempt_disable();  /* Prevent CPU hotplug from confusing us. */
+	ret = num_online_cpus() == 1;
+	preempt_enable();
+	return ret;
 }
 
 extern void rcu_scheduler_starting(void);
-- 
1.7.8