Re: [PATCH bpf-next] xsk: update rings for load-acquire/store-release semantics

[Will Deacon <will@kernel.org>]

On Tue, Mar 17, 2020 at 08:14:09PM +0100, Björn Töpel wrote:
> On Mon, 16 Mar 2020 at 19:44, Will Deacon <will@kernel.org> wrote:
> >
> > On Tue, Jan 21, 2020 at 12:50:23PM +0100, Björn Töpel wrote:
> > > On Tue, 21 Jan 2020 at 00:51, Daniel Borkmann <daniel@iogearbox.net> wrote:
> > > >
> > > > On 1/20/20 10:21 AM, Björn Töpel wrote:
> > > > > From: Björn Töpel <bjorn.topel@intel.com>
> > > > >
> > > > > Currently, the AF_XDP rings uses fences for the kernel-side
> > > > > produce/consume functions. By updating rings for
> > > > > load-acquire/store-release semantics, the full barrier (smp_mb()) on
> > > > > the consumer side can be replaced.
> > > > >
> > > > > Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
> > > >
> > > > If I'm not missing something from the ring update scheme, don't you also need
> > > > to adapt to STORE.rel ->producer with matching barrier in tools/lib/bpf/xsk.h ?
> > > >
> > >
> > > Daniel/John,
> > >
> > > Hmm, I was under the impression that *wasn't* the case. Quoting
> > > memory-barriers.txt:
> > >
> > > --8<--
> > > When dealing with CPU-CPU interactions, certain types of memory
> > > barrier should always be paired.  A lack of appropriate pairing is
> > > almost certainly an error.
> > >
> > > General barriers pair with each other, though they also pair with most
> > > other types of barriers, albeit without multicopy atomicity.  An
> > > acquire barrier pairs with a release barrier, but both may also pair
> > > with other barriers, including of course general barriers.  A write
> > > barrier pairs with a data dependency barrier, a control dependency, an
> > > acquire barrier, a release barrier, a read barrier, or a general
> > > barrier.  Similarly a read barrier, control dependency, or a data
> > > dependency barrier pairs with a write barrier, an acquire barrier, a
> > > release barrier, or a general barrier:
> > > -->8--
> >
> > The key part here is "albeit without multicopy atomicity". I don't think
> > you care about that at all for these rings as you're very clearly passing a
> > message from the producer side to the consumer side in a point-to-point like
> > manner, so I think you're ok to change the kernel independently from
> > userspace (but I would still recommend updating both eventually).
> >
> > The only thing you might run into is if anybody is relying on the smp_mb()
> > in the consumer to order other unrelated stuff either side of the consume
> > operation (or even another consume operation to a different ring!), but it
> > looks like you can't rely on that in the xsk queue implementation anyway
> > because you cache the global state and so the barriers are conditional.
> >
> 
> Thanks for getting back, and for the clarification! I'll do a respin
> (as part of a another series) that include the userland changes.

I'm just sorry it took me so long. I got tied up with personal stuff,
then conferences and finally, when it got to the top of my list, I took
some holiday.

Will