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

Z qiang
2024-02-27T06:40:07Z

>
> 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)

It seems that we should move rcu_sr_put_wait_head() from
rcu_sr_normal_gp_cleanup() to
rcu_sr_normal_gp_cleanup_work(), if find wait_head->next == NULL, invoke
rcu_sr_put_wait_head() to release wait_head.

Thanks
Zqiang

> ...
>
> // 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!!!
> }
> }
>