1 files changed, 8 insertions, 2 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9bb5dff50815..f431114bc06a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3684,8 +3684,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
          */
         rnp = rdp->mynode;
         raw_spin_lock_rcu_node(rnp);            /* irqs already disabled. */
-        if (!rdp->beenonline)
-                WRITE_ONCE(rsp->ncpus, READ_ONCE(rsp->ncpus) + 1);
         rdp->beenonline = true;  /* We have now been online. */
         rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
         rdp->completed = rnp->completed;
@@ -3789,6 +3787,8 @@ void rcu_cpu_starting(unsigned int cpu)
 {
         unsigned long flags;
         unsigned long mask;
+        int nbits;
+        unsigned long oldmask;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
         struct rcu_state *rsp;
@@ -3799,9 +3799,15 @@ void rcu_cpu_starting(unsigned int cpu)
                 mask = rdp->grpmask;
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
                 rnp->qsmaskinitnext |= mask;
+                oldmask = rnp->expmaskinitnext;
                 rnp->expmaskinitnext |= mask;
+                oldmask ^= rnp->expmaskinitnext;
+                nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
+                /* Allow lockless access for expedited grace periods. */
+                smp_store_release(&rsp->ncpus, rsp->ncpus + nbits); /* ^^^ */
                 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
         }
+        smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
 }
 
 #ifdef CONFIG_HOTPLUG_CPU