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

Frederic Weisbecker
2022-12-20T23:05:32Z

On Tue, Dec 20, 2022 at 07:06:57PM +0000, Joel Fernandes wrote:
> On Tue, Dec 20, 2022 at 7:01 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
> >
> > On 2022-12-20 13:29, Joel Fernandes wrote:
> > >
> >
> > > I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed…
> >
> > I strongly suspect the memory barrier after flip is useless for the same
> > reasons I mentioned explaining why the barrier before the flip is useless.
> >
> > However, we need to double-check that we have memory barriers at the
> > beginning and end of synchronize_srcu, and between load of "unlock"
> > counters and load of "lock" counters.
> >
> > Where is the barrier at the beginning of synchronize_srcu ?
>
> I believe we don't need another memory barrier at the beginning of
> synchronize_srcu() (but this part of my SRCU study is still pending
> ;)) . The grace period guarantee (read-side critical sections don't
> span the GP) is already enforced by the memory barrier between
> scanning for all unlocks, and scanning for all locks (Paired with
> corresponding memory barriers on the read-side).
>
> Accordingly, before we scan all locks and match lock == unlock, there
> is an smp_mb(). Why is that not sufficient?

That's not enough, you still need a barrier between the updater's pre-GP
accesses and the scans, so that post-GP read side sees the updater's pre-GP
accesses:

UPDATER READER
------- ------
WRITE A WRITE srcu_read_lock
smp_mb() //rcu_seq_snap() smp_mb()
READ srcu_read_lock //scans READ A

Thanks.

>
> Thanks,
>
> - Joel
>
> >
> > Thanks,
> >
> > Mathieu
> >
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > https://www.efficios.com
> >