rcu: Add WRITE_ONCE() to rcu_node ->boost_tasks

The rcu_node structure's ->boost_tasks field is read locklessly, so this
commit adds the WRITE_ONCE() to an update in order to provide proper
documentation and READ_ONCE()/WRITE_ONCE() pairing.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ed6bb460c7c6d212a8128dca4756e15d87173fa6..664d0aa48edb61c689738b8b92a0502a09854034 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -505,7 +505,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
                        /* Snapshot ->boost_mtx ownership w/rnp->lock held. */
                        drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
                        if (&t->rcu_node_entry == rnp->boost_tasks)
-                               rnp->boost_tasks = np;
+                               WRITE_ONCE(rnp->boost_tasks, np);
                }
 
                /*
@@ -1082,7 +1082,7 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
             rnp->qsmask == 0 &&
             (ULONG_CMP_GE(jiffies, rnp->boost_time) || rcu_state.cbovld))) {
                if (rnp->exp_tasks == NULL)
-                       rnp->boost_tasks = rnp->gp_tasks;
+                       WRITE_ONCE(rnp->boost_tasks, rnp->gp_tasks);
                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
                rcu_wake_cond(rnp->boost_kthread_task,
                              READ_ONCE(rnp->boost_kthread_status));