Re: [PATCH 1/2] rcu: Comment on the extraneous delta test on rcu_seq_done_exact()

[Joel Fernandes]

On Tue, Mar 18, 2025 at 11:37:38AM -0700, Paul E. McKenney wrote:
> On Tue, Mar 18, 2025 at 02:56:18PM +0100, Frederic Weisbecker wrote:
> > The numbers used in rcu_seq_done_exact() lack some explanation behind
> > their magic. Especially after the commit:
> > 
> >     85aad7cc4178 ("rcu: Fix get_state_synchronize_rcu_full() GP-start detection")
> > 
> > which reported a subtle issue where a new GP sequence snapshot was taken
> > on the root node state while a grace period had already been started and
> > reflected on the global state sequence but not yet on the root node
> > sequence, making a polling user waiting on a wrong already started grace
> > period that would ignore freshly online CPUs.
> > 
> > The fix involved taking the snaphot on the global state sequence and
> > waiting on the root node sequence. And since a grace period is first
> > started on the global state and only afterward reflected on the root
> > node, a snapshot taken on the global state sequence might be two full
> > grace periods ahead of the root node as in the following example:
> > 
> > rnp->gp_seq = rcu_state.gp_seq = 0
> > 
> >     CPU 0                                           CPU 1
> >     -----                                           -----
> >     // rcu_state.gp_seq = 1
> >     rcu_seq_start(&rcu_state.gp_seq)
> >                                                     // snap = 8
> >                                                     snap = rcu_seq_snap(&rcu_state.gp_seq)
> >                                                     // Two full GP differences
> >                                                     rcu_seq_done_exact(&rnp->gp_seq, snap)
> >     // rnp->gp_seq = 1
> >     WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq);
> > 
> > Add a comment about those expectations and to clarify the magic within
> > the relevant function.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> 
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> But it would of course be good to get reviews from the others.

I actually don't agree that the magic in the rcu_seq_done_exact() function about the
~2 GPs is related to the lag between rcu_state.gp_seq and root rnp->gp_seq,
because the small lag can just as well survive with the rcu_seq_done()
function in the above sequence right?

The rcu_seq_done_exact() function on the other hand is more about not being
stuck in the ULONG_MAX/2 guard band, but to actually get to that, you need a
wrap around to happen and the delta between "rnp->gp_seq" and "snap" to be at
least ULONG_MAX/2 AFAIU.

So the only time this magic will matter is if you have a huge delta between
what is being compared, not just 2 GPs.

Or, did I miss something?

(Also sorry about slow email responses this week as I had my presentation
today and was busy preparing this week and attending other presentations at
OSPM, I'll provide an update on that soon!).

thanks,

 - Joel









> 
> > ---
> >  kernel/rcu/rcu.h | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > index eed2951a4962..7acf1f36dd6c 100644
> > --- a/kernel/rcu/rcu.h
> > +++ b/kernel/rcu/rcu.h
> > @@ -157,6 +157,13 @@ static inline bool rcu_seq_done(unsigned long *sp, unsigned long s)
> >   * Given a snapshot from rcu_seq_snap(), determine whether or not a
> >   * full update-side operation has occurred, but do not allow the
> >   * (ULONG_MAX / 2) safety-factor/guard-band.
> > + *
> > + * The token returned by get_state_synchronize_rcu_full() is based on
> > + * rcu_state.gp_seq but it is tested in poll_state_synchronize_rcu_full()
> > + * against the root rnp->gp_seq. Since rcu_seq_start() is first called
> > + * on rcu_state.gp_seq and only later reflected on the root rnp->gp_seq,
> > + * it is possible that rcu_seq_snap(rcu_state.gp_seq) returns 2 full grace
> > + * periods ahead of the root rnp->gp_seq.
> >   */
> >  static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
> >  {
> > -- 
> > 2.48.1
> >