Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

Joel Fernandes
2022-12-18T21:30:53Z

Hi Mathieu,

On Sun, Dec 18, 2022 at 3:56 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> On 2022-12-18 14:13, Joel Fernandes (Google) wrote:
> > Hello, I believe the pre-flip memory barrier is not required. The only reason I
> > can say to remove it, other than the possibility that it is unnecessary, is to
> > not have extra code that does not help. However, since we are issuing a fully
> > memory-barrier after the flip, I cannot say that it hurts to do it anyway.
> >
> > For this reason, please consider these patches as "informational", than a
> > "please merge". :-) Though, feel free to consider merging if you agree!
> >
> > All SRCU scenarios pass with these, with 6 hours of testing.
>
> Hi Joel,
>
> Please have a look at the comments in my side-rcu implementation [1, 2].
> It is similar to what SRCU does (per-cpu counter based grace period
> tracking), but implemented for userspace. The comments explain why this
> works without the memory barrier you identify as useless in SRCU.
>
> Following my implementation of side-rcu, I reviewed the SRCU comments
> and identified that the barrier "/* E */" appears to be useless. I even
> discussed this privately with Paul E. McKenney.
>
> My implementation and comments go further though, and skip the period
> "flip" entirely if the first pass observes that all readers (in both
> periods) are quiescent.

Actually in SRCU, the first pass scans only 1 index, then does the
flip, and the second pass scans the second index. Without doing a
flip, an index cannot be scanned for forward progress reasons because
it is still "active". So I am curious how you can skip flip and still
scan both indexes? I will dig more into your implementation to learn more.

> The most relevant comment in side-rcu is:
>
> * The grace period completes when it observes that there are no active
> * readers within each of the periods.
> *
> * The active_readers state is initially true for each period, until the
> * grace period observes that no readers are present for each given
> * period, at which point the active_readers state becomes false.
>
> So I agree with the clarifications you propose here, but I think we can
> improve the grace period implementation further by clarifying the SRCU
> grace period model.

Thanks a lot, I am curious how you do the "detection of no new
readers" part without globally doing some kind of synchronization. I
will dig more into your implementation to learn more.

Thanks,

- Joel

>
> Thanks,
>
> Mathieu
>
>
> [1] https://github.com/efficios/libside/blob/master/src/rcu.h
> [2] https://github.com/efficios/libside/blob/master/src/rcu.c
>
> >
> > thanks,
> >
> > - Joel
> >
> > Joel Fernandes (Google) (2):
> > srcu: Remove comment about prior read lock counts
> > srcu: Remove memory barrier "E" as it is not required
> >
> > kernel/rcu/srcutree.c | 10 ----------
> > 1 file changed, 10 deletions(-)
> >
> > --
> > 2.39.0.314.g84b9a713c41-goog
> >
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>