linux-kernel - Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize

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Message-ID: <Z8bbary51FZ2TsUA@pc636>
Date: Tue, 4 Mar 2025 11:52:26 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joelagnelf@...dia.com>
Cc: Joel Fernandes <joelagnelf@...dia.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Uladzislau Rezki <urezki@...il.com>, RCU <rcu@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Cheung Wall <zzqq0103.hey@...il.com>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

> > 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@...lfernandes.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.

--
Uladzislau Rezki