[PATCH RFC tip/core/rcu 5/6] rcu: Apply ACCESS_ONCE() to rcu_boost() return value

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

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

Both TINY_RCU's and TREE_RCU's implementations of rcu_boost() access
the ->boost_tasks and ->exp_tasks fields without preventing concurrent
changes to these fields.  This commit therefore applies ACCESS_ONCE in
order to prevent compiler mischief.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcutiny_plugin.h |    4 ++--
 kernel/rcutree_plugin.h |    3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index dfa97cb..9cb1ae4 100644
--- a/kernel/rcutiny_plugin.h
+++ b/kernel/rcutiny_plugin.h
@@ -312,8 +312,8 @@ static int rcu_boost(void)
 	rt_mutex_lock(&mtx);
 	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
 
-	return rcu_preempt_ctrlblk.boost_tasks != NULL ||
-	       rcu_preempt_ctrlblk.exp_tasks != NULL;
+	return ACCESS_ONCE(rcu_preempt_ctrlblk.boost_tasks) != NULL ||
+	       ACCESS_ONCE(rcu_preempt_ctrlblk.exp_tasks) != NULL;
 }
 
 /*
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index f55f10b..8bb35d7 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1232,7 +1232,8 @@ static int rcu_boost(struct rcu_node *rnp)
 	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
 	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
 
-	return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
+	return ACCESS_ONCE(rnp->exp_tasks) != NULL ||
+	       ACCESS_ONCE(rnp->boost_tasks) != NULL;
 }
 
 /*
-- 
1.7.8