rcu: Add READ_ONCE and data_race() to rcu_node ->boost_tasks

The rcu_node structure's ->boost_tasks field is read locklessly, so this
commit adds the READ_ONCE() to one load in order to avoid destructive
compiler optimizations.  The other load is from a diagnostic print,
so data_race() suffices.

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 35d77db035bd564fdb45bc10b266ccb39ab6b89a..ed6bb460c7c6d212a8128dca4756e15d87173fa6 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -760,7 +760,7 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
                pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
                        __func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
        pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
-               __func__, READ_ONCE(rnp->gp_tasks), rnp->boost_tasks,
+               __func__, READ_ONCE(rnp->gp_tasks), data_race(rnp->boost_tasks),
                READ_ONCE(rnp->exp_tasks));
        pr_info("%s: ->blkd_tasks", __func__);
        i = 0;
@@ -1036,7 +1036,8 @@ static int rcu_boost_kthread(void *arg)
        for (;;) {
                WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
                trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
-               rcu_wait(rnp->boost_tasks || READ_ONCE(rnp->exp_tasks));
+               rcu_wait(READ_ONCE(rnp->boost_tasks) ||
+                        READ_ONCE(rnp->exp_tasks));
                trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
                WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
                more2boost = rcu_boost(rnp);