Hi, Paul and Frederic,
[...]
> > > On Sat, Jan 25, 2025 at 12:03:58AM +0100, Frederic Weisbecker wrote:
> > > > Le Fri, Dec 13, 2024 at 11:49:49AM -0800, Paul E. McKenney a écrit :
> > > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> > > > > index 2f9c9272cd486..d2a91f705a4ab 100644
> > > > > --- a/kernel/rcu/rcu.h
> > > > > +++ b/kernel/rcu/rcu.h
> > > > > @@ -162,7 +162,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
> > > > > {
> > > > > unsigned long cur_s = READ_ONCE(*sp);
> > > > >
> > > > > - return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 * RCU_SEQ_STATE_MASK + 1));
> > > > > + return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 * RCU_SEQ_STATE_MASK + 1));
> > > >
[...]
> > > > The way I understand it is that rcu_state.gp_seq might be seen started while
> > > > root_rnp->gp_seq is not. So rcu_seq_snap() on the started rcu_state.gp_seq
> > > > may return maximum 2 full GPs ahead of root_rnp->gp_seq. And therefore it takes below
> > > > 2 GPs to safely deduce we wrapped around.
> > >
> > > Exactly!
> > >
> > > > Should it be ULONG_CMP_LT(cur_s, s - (2 * (RCU_SEQ_STATE_MASK + 1))) ?
> > >
> > > Quite possibly. I freely admit that I allowed a bit of slop because
> > > time was of the essence (holidays and all that) and also it does not
> > > hurt much to lose a couple of counts out of a 2^32 cycle, to say nothing
> > > of the common-case 2^64 cycle. It would not hurt to be exact, but it
> > > would be necessary to convince ourselves that we were not off by one in
> > > the wrong direction.
> > >
> > > I would be happy to see a patch, as long as it was sufficiently
> > > convincing.
> >
> > I'm not so much concerned about being exact but rather about making
> > sure we still understand what we did within one year. We can leave one
> > more grace period than what we expect out of paranoia but, the most
> > important is that we comment about what we expect and why. Let me
> > prepare a patch for that.
>
> Even better!
Do we really have users who could pass such a huge delta wrapped
around value to poll() i.e > ULONG_MAX/2 ? For 32-bit, that would be
2 billion count since the get() (500 million GPs on 32-bit?). I am
curious if such a scenario should be a WARN() because also: If more
than ULONG_MAX/2 values are possible after get(), then a full or
multiple ULONG_MAX wraparound is also possible. This means then both
rcu_seq_done() and rcu_seq_done_exact() become unreliable anyway for
such stale get() values.
I do agree with both your points on the side effect of the patch to
rcu_seq_done_exact(), but I am not fully convinced myself about
utility of rcu_seq_done_exact() versus the rcu_seq_done() but I could
be missing something.
Otherwise I analyzed the patch and it makes sense to me:
Reviewed-by:Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
thanks,
- Joel
