On Tue, Mar 04, 2025 at 11:52:26AM +0100, Uladzislau Rezki wrote:
> > > Did I get that right?
> > >
> >
> > Other than I'm unable to follow what do you mean "WH has not been
> > injected, so nothing to wait on", maybe because I am missing some
> > terminology from you ;-) I think it's a good analysis, thank you!
> >
> > > I think this is a real bug AFAICS, hoping all the memory barriers are in
> > > place to make sure the code reordering also correctly orders the accesses.
> > > I'll double check that.
> > >
> > > I also feel its 'theoretical', because as long as rcu_gp_init() and
> > > rcu_gp_cleanup() are properly ordered WRT pre-existing readers, then
> > > synchronize_rcu_normal() still waits for pre-existing readers even though its
> > > a bit confused about the value of the cookies.
> > >
> > > For the fix,
> > > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > >
> > > (If possible, include a Link: to my (this) post so that the sequence of
> > > events is further clarified.)
> > >
> >
> > Will add the tag (with the email you really want ;-)) and a link to this
> > email to the patch. Thanks!
> >
>
> CPU_1: | CPU_2:
> # Increase a seq-number |
> rcu_seq_start(&rcu_state.gp_seq); |
> | add_client() {
> | # Record a gp-sec state
> | get_state_synchronize_rcu_full(&rs.oldstate);
> | }
> |
> | rcu_sr_normal_gp_init() {
> | add a dummy-wait-head;
> | }
>
>
> A client has been added with already updated gp-sec number, i.e.
> "oldstate" would refer to this GP, not to previous. A poll_state_synchronize_rcu_full()
> will complain because this GP is not passed, it will on a next iteration.
>
> This is how i see this.
>
Updated the plain-text, removed tabs:
CPU_1: | CPU_2:
# Increase a seq-number |
rcu_seq_start(&rcu_state.gp_seq); |
| add_client() {
| # Record a gp-sec state
| get_state_synchronize_rcu_full(&rs.oldstate);
| }
|
| rcu_sr_normal_gp_init() {
| add a dummy-wait-head;
| }
--
Uladzislau Rezki
