Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

[Uladzislau Rezki <urezki@gmail.com>]

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@joelfernandes.org>
> > > 
> > > (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