rcu: Add WRITE_ONCE() to rcu_node ->qsmaskinitnext

The rcu_state structure's ->qsmaskinitnext 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 for systems not doing incessant CPU-hotplug
operations.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9443909..346321a 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3379,7 +3379,7 @@ void rcu_cpu_starting(unsigned int cpu)
        rnp = rdp->mynode;
        mask = rdp->grpmask;
        raw_spin_lock_irqsave_rcu_node(rnp, flags);
-       rnp->qsmaskinitnext |= mask;
+       WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
        oldmask = rnp->expmaskinitnext;
        rnp->expmaskinitnext |= mask;
        oldmask ^= rnp->expmaskinitnext;
@@ -3432,7 +3432,7 @@ void rcu_report_dead(unsigned int cpu)
                rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
        }
-       rnp->qsmaskinitnext &= ~mask;
+       WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);
        raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
        raw_spin_unlock(&rcu_state.ofl_lock);