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

[Uladzislau Rezki <urezki@gmail.com>]

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.

Thanks!

--
Uladzislau Rezki