rcu: Add WRITE_ONCE() to rcu_node ->qsmask update

The rcu_node structure's ->qsmask 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.c b/kernel/rcu/tree.c
index d91c915..bb57c24 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1881,7 +1881,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
                WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */
                WARN_ON_ONCE(!rcu_is_leaf_node(rnp) &&
                             rcu_preempt_blocked_readers_cgp(rnp));
-               rnp->qsmask &= ~mask;
+               WRITE_ONCE(rnp->qsmask, rnp->qsmask & ~mask);
                trace_rcu_quiescent_state_report(rcu_state.name, rnp->gp_seq,
                                                 mask, rnp->qsmask, rnp->level,
                                                 rnp->grplo, rnp->grphi,