Re: [dpdk-dev] [PATCH v3 1/3] ring: read tail using atomic load

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> >         >
> >         > Hi Jerin,
> >         >
> >         > Thanks for your review, inline comments from our internal
> discussions.
> >         >
> >         > BR. Gavin
> >         >
> >         > > -----Original Message-----
> >         > > From: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> >         > > Sent: Saturday, September 29, 2018 6:49 PM
> >         > > To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>
> >         > > Cc: dev@dpdk.org; Honnappa Nagarahalli
> >         > > <Honnappa.Nagarahalli@arm.com>; Steve Capper
> >         > > <Steve.Capper@arm.com>; Ola Liljedahl <Ola.Liljedahl@arm.com>;
> nd
> >         > > <nd@arm.com>; stable@dpdk.org
> >         > > Subject: Re: [PATCH v3 1/3] ring: read tail using atomic load
> >         > >
> >         > > -----Original Message-----
> >         > > > Date: Mon, 17 Sep 2018 16:17:22 +0800
> >         > > > From: Gavin Hu <gavin.hu@arm.com>
> >         > > > To: dev@dpdk.org
> >         > > > CC: gavin.hu@arm.com, Honnappa.Nagarahalli@arm.com,
> >         > > > steve.capper@arm.com,  Ola.Liljedahl@arm.com,
> >         > > > jerin.jacob@caviumnetworks.com, nd@arm.com,
> stable@dpdk.org
> >         > > > Subject: [PATCH v3 1/3] ring: read tail using atomic load
> >         > > > X-Mailer: git-send-email 2.7.4
> >         > > >
> >         > > > External Email
> >         > > >
> >         > > > In update_tail, read ht->tail using __atomic_load.Although the
> >         > > > compiler currently seems to be doing the right thing even without
> >         > > > _atomic_load, we don't want to give the compiler freedom to
> optimise
> >         > > > what should be an atomic load, it should not be arbitarily moved
> >         > > > around.
> >         > > >
> >         > > > Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier
> option")
> >         > > > Cc: stable@dpdk.org
> >         > > >
> >         > > > Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> >         > > > Reviewed-by: Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>
> >         > > > Reviewed-by: Steve Capper <steve.capper@arm.com>
> >         > > > Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
> >         > > > ---
> >         > > >  lib/librte_ring/rte_ring_c11_mem.h | 3 ++-
> >         > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> >         > > >
> >         > The read of ht->tail needs to be atomic, a non-atomic read would not
> be correct.
> >
> >         That's a 32bit value load.
> >         AFAIK on all CPUs that we support it is an atomic operation.
> >     [Ola] But that the ordinary C load is translated to an atomic load for the
> target architecture is incidental.
> >
> >     If the design requires an atomic load (which is the case here), we
> > should use an atomic load on the language level. Then we can be sure it will
> always be translated to an atomic load for the target in question or
> compilation will fail. We don't have to depend on assumptions.
> 
> We all know that 32bit load/store on cpu we support - are atomic.
> If it wouldn't be the case - DPDK would be broken in dozen places.
> So what the point to pretend that "it might be not atomic" if we do know for
> sure that it is?
> I do understand that you want to use atomic_load(relaxed) here for
> consistency, and to conform with C11 mem-model and I don't see any harm in
> that.
We can continue to discuss the topic, it is a good discussion. But, as far this patch is concerned, can I consider this as us having a consensus? The file rte_ring_c11_mem.h is specifically for C11 memory model and I also do not see any harm in having code that completely conforms to C11 memory model.

> But argument that we shouldn't assume 32bit load/store ops as atomic
> sounds a bit flaky to me.
> Konstantin
> 
> 
> >
> >
> >
> >         > But there are no memory ordering requirements (with
> >         > regards to other loads and/or stores by this thread) so relaxed
> memory order is sufficient.
> >         > Another aspect of using __atomic_load_n() is that the
> > compiler cannot "optimise" this load (e.g. combine, hoist etc), it has to be
> done as
> >         > specified in the source code which is also what we need here.
> >
> >         I think Jerin points that rte_pause() acts here as compiler barrier too,
> >         so no need to worry that compiler would optimize out the loop.
> >     [Ola] Sorry missed that. But the barrier behaviour of rte_pause()
> > is not part of C11, is it essentially a hand-made feature to support
> > the legacy multithreaded memory model (which uses explicit HW and
> compiler barriers). I'd prefer code using the C11 memory model not to
> depend on such legacy features.
> >
> >
> >
> >         Konstantin
> >
> >         >
> >         > One point worth mentioning though is that this change is for
> > the rte_ring_c11_mem.h file, not the legacy ring. It may be worth persisting
> >         > with getting the C11 code right when people are less excited about
> sending a release out?
> >         >
> >         > We can explain that for C11 we would prefer to do loads and stores
> as per the C11 memory model. In the case of rte_ring, the code is
> >         > separated cleanly into C11 specific files anyway.
> >         >
> >         > I think reading ht->tail using __atomic_load_n() is the most
> appropriate way. We show that ht->tail is used for synchronization, we
> >         > acknowledge that ht->tail may be written by other threads
> > without any other kind of synchronization (e.g. no lock involved) and we
> require
> >         > an atomic load (any write to ht->tail must also be atomic).
> >         >
> >         > Using volatile and explicit compiler (or processor) memory barriers
> (fences) is the legacy pre-C11 way of accomplishing these things.
> > There's
> >         > a reason why C11/C++11 moved away from the old ways.
> >         > > >
> >         > > >         __atomic_store_n(&ht->tail, new_val, __ATOMIC_RELEASE);
> >         > > > --
> >         > > > 2.7.4
> >         > > >
> >
> >
> >