rcu: Add WRITE_ONCE() to rcu_state ->gp_req_activity

The rcu_state structure's ->gp_req_activity 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 236692d..9443909 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1200,7 +1200,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
        }
        trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
        WRITE_ONCE(rcu_state.gp_flags, rcu_state.gp_flags | RCU_GP_FLAG_INIT);
-       rcu_state.gp_req_activity = jiffies;
+       WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
        if (!rcu_state.gp_kthread) {
                trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
                goto unlock_out;
@@ -1775,7 +1775,7 @@ static void rcu_gp_cleanup(void)
                    rcu_segcblist_is_offloaded(&rdp->cblist);
        if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) {
                WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
-               rcu_state.gp_req_activity = jiffies;
+               WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
                trace_rcu_grace_period(rcu_state.name,
                                       READ_ONCE(rcu_state.gp_seq),
                                       TPS("newreq"));