On Thu, Oct 19, 2023 at 07:44:32PM +0800, Hillf Danton wrote:
> On Wed, 18 Oct 2023 13:20:49 +0200 Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> wrote:
> > > On Mon, Oct 16, 2023 at 1:30 PM Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> wrote:
> > > > +static void rcu_sr_normal_gp_init(void)
> > > > +{
> > > > + struct llist_node *llnode, *rcu;
> > > > + int ret;
> > > > +
> > > > + if (llist_empty(&sr.curr))
> > > > + return;
> > >
> > > This empty check erases the curr_tail race below instead of
> > > atomic_inc_return(&sr.active), because llist_add() will never return true
> > > after this check.
> > >
> > I use "active" counter to guarantee that a tail was updated in the
> > rcu_sr_normal_add_req(), i.e. the list might be not empty whereas the
> > tail updating might be in progress. llist_add() success and the task gets
> > preemted as an example.
>
> You are right - the preempt is what I missed.
>
> Then another question rising - the adding order of sync requests is changed
> at wakeup time, as shown by the two functions below with sr.curr_tail and
> sr.active cut off.
>
> static void rcu_sr_normal_gp_init(void)
> {
> struct llist_node *llnode, *rcu;
>
> llnode = llist_del_all(&sr.curr);
> rcu = llist_reverse_order(llnode);
> if (!rcu)
> return;
> /*
> * A waiting list of GP should be empty on this step,
> * since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
> * rolls it over. If not, it is a BUG, warn a user.
> */
> WARN_ON_ONCE(!llist_empty(&sr.wait));
>
> WRITE_ONCE(sr.wait_tail, llnode);
>
> llist_add_batch(rcu, llnode, &sr.wait);
> }
>
> static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> {
> llist_add((struct llist_node *) &rs->head, &sr.curr);
> }
>
This is what i was thinking of in the beginning but i had decided
to make it more complex and maintain the current tail. We will get
a slight penalty in performance on a synthetic test but i have compared
it and this is negligible in fact.
Another good thing with it is we process users in a reverse order,
i.e. the most waiting users to less ones, this is from a waiting time
point of view.
I like it better and it is more simple. Anyway we can improve it
further.
Thank you for the proposal!
--
Uladzislau Rezki
