rcu: Mark accesses to rcu_state.n_force_qs

This commit marks accesses to the rcu_state.n_force_qs.  These data
races are hard to make happen, but syzkaller was equal to the task.

Reported-by: syzbot+e08a83a1940ec3846cd5@syzkaller.appspotmail.com
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bce848e..c89f5e6 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1907,7 +1907,7 @@ static void rcu_gp_fqs(bool first_time)
        struct rcu_node *rnp = rcu_get_root();
 
        WRITE_ONCE(rcu_state.gp_activity, jiffies);
-       rcu_state.n_force_qs++;
+       WRITE_ONCE(rcu_state.n_force_qs, rcu_state.n_force_qs + 1);
        if (first_time) {
                /* Collect dyntick-idle snapshots. */
                force_qs_rnp(dyntick_save_progress_counter);
@@ -2550,7 +2550,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
        /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
        if (count == 0 && rdp->qlen_last_fqs_check != 0) {
                rdp->qlen_last_fqs_check = 0;
-               rdp->n_force_qs_snap = rcu_state.n_force_qs;
+               rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
        } else if (count < rdp->qlen_last_fqs_check - qhimark)
                rdp->qlen_last_fqs_check = count;
 
@@ -2898,10 +2898,10 @@ static void __call_rcu_core(struct rcu_data *rdp, struct rcu_head *head,
                } else {
                        /* Give the grace period a kick. */
                        rdp->blimit = DEFAULT_MAX_RCU_BLIMIT;
-                       if (rcu_state.n_force_qs == rdp->n_force_qs_snap &&
+                       if (READ_ONCE(rcu_state.n_force_qs) == rdp->n_force_qs_snap &&
                            rcu_segcblist_first_pend_cb(&rdp->cblist) != head)
                                rcu_force_quiescent_state();
-                       rdp->n_force_qs_snap = rcu_state.n_force_qs;
+                       rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
                        rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
                }
        }
@@ -4128,7 +4128,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
        /* Set up local state, ensuring consistent view of global state. */
        raw_spin_lock_irqsave_rcu_node(rnp, flags);
        rdp->qlen_last_fqs_check = 0;
-       rdp->n_force_qs_snap = rcu_state.n_force_qs;
+       rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
        rdp->blimit = blimit;
        rdp->dynticks_nesting = 1;      /* CPU not up, no tearing. */
        rcu_dynticks_eqs_online();