rcu: add rcu_read_lock_sched() / rcu_read_unlock_sched()

Add rcu_read_lock_sched() and rcu_read_unlock_sched() to rcupdate.h to match the
recently added write-side call_rcu_sched() and rcu_barrier_sched(). They also
match the no-so-recently-added synchronize_sched().

It will help following matching use of the update/read lock primitives. Those
new read lock will replace preempt_disable()/enable() used in pair with
RCU-classic synchronization.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/rcupdate.h

@@ -132,6 +132,26 @@ struct rcu_head {
  */
 #define rcu_read_unlock_bh() __rcu_read_unlock_bh()
 
+/**
+ * rcu_read_lock_sched - mark the beginning of a RCU-classic critical section
+ *
+ * Should be used with either
+ * - synchronize_sched()
+ * or
+ * - call_rcu_sched() and rcu_barrier_sched()
+ * on the write-side to insure proper synchronization.
+ */
+#define rcu_read_lock_sched() preempt_disable()
+
+/*
+ * rcu_read_unlock_sched - marks the end of a RCU-classic critical section
+ *
+ * See rcu_read_lock_sched for more information.
+ */
+#define rcu_read_unlock_sched() preempt_enable()
+
+
+
 /**
  * rcu_dereference - fetch an RCU-protected pointer in an
  * RCU read-side critical section.  This pointer may later