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

Frederic Weisbecker
2024-03-04T11:55:50Z

Le Wed, Feb 28, 2024 at 07:04:21PM +0100, Uladzislau Rezki a écrit :
> On Tue, Feb 27, 2024 at 12:07:32AM +0100, Frederic Weisbecker wrote:
> > 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!!!
> > }
> > }
> Looks like that. To address this, we should not release the head in the GP
> > kthread.

But then you have to unconditionally schedule the work, right? Otherwise the
HEADs are not released. And that means dropping this patch (right now I don't
have a better idea).