Re: [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency

Frederic Weisbecker
2024-02-26T23:07:34Z

On Tue, Feb 20, 2024 at 07:31:13PM +0100, Uladzislau Rezki (Sony) wrote:
> +static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> +{
> + struct llist_node *done, *rcu, *next, *head;
> +
> + /*
> + * This work execution can potentially execute
> + * while a new done tail is being updated by
> + * grace period kthread in rcu_sr_normal_gp_cleanup().
> + * So, read and updates of done tail need to
> + * follow acq-rel semantics.
> + *
> + * Given that wq semantics guarantees that a single work
> + * cannot execute concurrently by multiple kworkers,
> + * the done tail list manipulations are protected here.
> + */
> + done = smp_load_acquire(&rcu_state.srs_done_tail);
> + if (!done)
> + return;
> +
> + WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
> + head = done->next;
> + done->next = NULL;

Can the following race happen?

CPU 0 CPU 1
----- -----

// wait_tail == HEAD1
rcu_sr_normal_gp_cleanup() {
// has passed SR_MAX_USERS_WAKE_FROM_GP
wait_tail->next = next;
// done_tail = HEAD1
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
queue_work() {
test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
__queue_work()
}
}

set_work_pool_and_clear_pending()
rcu_sr_normal_gp_cleanup_work() {
// new GP, wait_tail == HEAD2
rcu_sr_normal_gp_cleanup() {
// executes all completion, but stop at HEAD1
wait_tail->next = HEAD1;
// done_tail = HEAD2
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
queue_work() {
test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
__queue_work()
}
}
// done = HEAD2
done = smp_load_acquire(&rcu_state.srs_done_tail);
// head = HEAD1
head = done->next;
done->next = NULL;
llist_for_each_safe() {
// completes all callbacks, release HEAD1
}
}
// Process second queue
set_work_pool_and_clear_pending()
rcu_sr_normal_gp_cleanup_work() {
// done = HEAD2
done = smp_load_acquire(&rcu_state.srs_done_tail);

// new GP, wait_tail == HEAD3
rcu_sr_normal_gp_cleanup() {
// Finds HEAD2 with ->next == NULL at the end
rcu_sr_put_wait_head(HEAD2)
...

// A few more GPs later
rcu_sr_normal_gp_init() {
HEAD2 = rcu_sr_get_wait_head();
llist_add(HEAD2, &rcu_state.srs_next);
// head == rcu_state.srs_next
head = done->next;
done->next = NULL;
llist_for_each_safe() {
// EXECUTE CALLBACKS TOO EARLY!!!
}
}