[PATCH tip/core/rcu 07/19] rcu: Add WRITE_ONCE() to rcu_node ->boost_tasks

paulmck
2020-04-15T17:11:17Z

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>
---
kernel/rcu/tree_plugin.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ed6bb46..664d0aa 100644
--- 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));
--
2.9.5