rcu/kvfree: Move need_offload_krc() out of krcp->lock

The need_offload_krc() function currently holds the krcp->lock in order
to safely check krcp->head.  This commit removes the need for this lock
in that function by updating the krcp->head pointer using WRITE_ONCE()
macro so that readers can carry out lockless loads of that pointer.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 839e617..0c42fce 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3194,7 +3194,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
                        // objects queued on the linked list.
                        if (!krwp->head_free) {
                                krwp->head_free = krcp->head;
-                               krcp->head = NULL;
+                               WRITE_ONCE(krcp->head, NULL);
                        }
 
                        WRITE_ONCE(krcp->count, 0);
@@ -3208,6 +3208,8 @@ static void kfree_rcu_monitor(struct work_struct *work)
                }
        }
 
+       raw_spin_unlock_irqrestore(&krcp->lock, flags);
+
        // If there is nothing to detach, it means that our job is
        // successfully done here. In case of having at least one
        // of the channels that is still busy we should rearm the
@@ -3215,8 +3217,6 @@ static void kfree_rcu_monitor(struct work_struct *work)
        // still in progress.
        if (need_offload_krc(krcp))
                schedule_delayed_monitor_work(krcp);
-
-       raw_spin_unlock_irqrestore(&krcp->lock, flags);
 }
 
 static enum hrtimer_restart
@@ -3386,7 +3386,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 
                head->func = ptr;
                head->next = krcp->head;
-               krcp->head = head;
+               WRITE_ONCE(krcp->head, head);
                success = true;
        }
 
@@ -3463,15 +3463,12 @@ static struct shrinker kfree_rcu_shrinker = {
 void __init kfree_rcu_scheduler_running(void)
 {
        int cpu;
-       unsigned long flags;
 
        for_each_possible_cpu(cpu) {
                struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
-               raw_spin_lock_irqsave(&krcp->lock, flags);
                if (need_offload_krc(krcp))
                        schedule_delayed_monitor_work(krcp);
-               raw_spin_unlock_irqrestore(&krcp->lock, flags);
        }
 }