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

Frederic Weisbecker
2024-02-27T16:16:30Z

Le Tue, Feb 27, 2024 at 03:37:26PM +0100, Frederic Weisbecker a écrit :
> Le Tue, Feb 27, 2024 at 02:39:55PM +0800, Z qiang a écrit :
> > > 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.
>
> Well, rcu_sr_normal_gp_cleanup_work() already put all the wait heads
> that are _after_ srs_done_tail. But it can't put the srs_done_tail itself
> without introducing even worse races...
>

(I forgot to mention this race actually concerns the last patch (4/4))