rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store

The rcu_node structure's ->exp_seq_rq field is read locklessly, so
this commit adds the WRITE_ONCE() to a load 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_exp.h b/kernel/rcu/tree_exp.h
index d7e04849c7ab7746e766b6e2cd7ed4a9d0074cb2..85b009e05637ff0282c68e2f05b450f6999fbd4f 100644 (file)
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s)
                                   sync_exp_work_done(s));
                        return true;
                }
-               rnp->exp_seq_rq = s; /* Followers can wait on us. */
+               WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
                spin_unlock(&rnp->exp_lock);
                trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
                                          rnp->grplo, rnp->grphi, TPS("nxtlvl"));