rcu: Add READ_ONCE() to rcu_data ->gpwrap

The rcu_data structure's ->gpwrap field is read locklessly, and so
this commit adds the required READ_ONCE() to a pair of laods in order
to avoid destructive compiler optimizations.

This data race was reported by KCSAN.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a70f56b..e851a12 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1322,7 +1322,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp,
 
        rcu_lockdep_assert_cblist_protected(rdp);
        c = rcu_seq_snap(&rcu_state.gp_seq);
-       if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
+       if (!READ_ONCE(rdp->gpwrap) && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
                /* Old request still live, so mark recent callbacks. */
                (void)rcu_segcblist_accelerate(&rdp->cblist, c);
                return;

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 43dc688..bca637b 100644 (file)
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -602,7 +602,7 @@ void show_rcu_gp_kthreads(void)
                        continue;
                for_each_leaf_node_possible_cpu(rnp, cpu) {
                        rdp = per_cpu_ptr(&rcu_data, cpu);
-                       if (rdp->gpwrap ||
+                       if (READ_ONCE(rdp->gpwrap) ||
                            ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
                                         READ_ONCE(rdp->gp_seq_needed)))
                                continue;