[PATCH v2 3/4] rcu/kvfree: Move need_offload_krc() out of krcp->lock

Uladzislau Rezki (Sony)
2022-11-29T16:00:33Z

Currently a need_offload_krc() function requires the krcp->lock
to be held because krcp->head can not be checked concurrently.

Fix it by updating the krcp->head using WRITE_ONCE() macro so
it becomes lock-free and safe for readers to see a valid data
without any locking.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
kernel/rcu/tree.c | 11 ++++-------
1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 445f8c11a9a3..c94c17194299 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3058,7 +3058,7 @@ 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);
@@ -3072,6 +3072,8 @@ 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
@@ -3079,8 +3081,6 @@ 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
@@ -3250,7 +3250,7 @@ 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;
}

@@ -3327,15 +3327,12 @@ 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);
}
}

--
2.30.2