Re: [dpdk-dev] [v1] ring: enforce reading the tails before ring operations

["Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>]

Hi ilya,

Thanks for your comments, inline comments and new commit message in V2.

/Gavin

> -----Original Message-----
> From: Ilya Maximets <i.maximets@samsung.com>
> Sent: Wednesday, March 6, 2019 7:49 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>;
> dev@dpdk.org
> Cc: nd <nd@arm.com>; thomas@monjalon.net; jerinj@marvell.com;
> hemant.agrawal@nxp.com; Nipun.gupta@nxp.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; olivier.matz@6wind.com
> Subject: Re: [v1] ring: enforce reading the tails before ring operations
> 
> On 06.03.2019 6:07, gavin hu wrote:
> > In weak memory models, like arm64, reading the {prod,cons}.tail may get
> > reordered after reading or writing the ring slots, which corrupts the ring
> > and stale data is observed.
> > This issue was reported by NXP on 8-A72 DPAA2 board. The problem is
> most
> > likely caused by missing the acquire semantics when reading cons.tail (in
> > SP enqueue) or prod.tail (in SC dequeue) which makes it possible to read
> a
> > stale value from the ring slots.  There must be a read fence before
> writing
> 
> Sorry, but the phrase "There must be a read fence before writing" makes
> no sense.
> Could you please rephrase or describe in details which reads you're trying
> to
> keep in exact order?

This patch is to keep in exact order for reading the tails(to calculate the available
/free slots of ring when moving the heads) before reading or writing the ring slots.

I rephrased the commit message in V2, could you have a look? 

> > or reading the ring slots, rte_atomic32_cmpset() provides the same
> ordering
> > for MP (and MC) case. This patch is to enforce this ordering for SP (and
> > SC) case.
> >
> > Signed-off-by: gavin hu <gavin.hu@arm.com>
> > Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
> > Tested-by: Nipun Gupta <nipun.gupta@nxp.com>
> > ---
> >  lib/librte_ring/rte_ring_generic.h | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/librte_ring/rte_ring_generic.h
> b/lib/librte_ring/rte_ring_generic.h
> > index ea7dbe5..1bd3dfd 100644
> > --- a/lib/librte_ring/rte_ring_generic.h
> > +++ b/lib/librte_ring/rte_ring_generic.h
> > @@ -90,9 +90,11 @@ __rte_ring_move_prod_head(struct rte_ring *r,
> unsigned int is_sp,
> >  			return 0;
> >
> >  		*new_head = *old_head + n;
> > -		if (is_sp)
> > -			r->prod.head = *new_head, success = 1;
> > -		else
> > +		if (is_sp) {
> > +			r->prod.head = *new_head;
> > +			rte_smp_rmb();
> > +			success = 1;
> > +		} else
> >  			success = rte_atomic32_cmpset(&r->prod.head,
> >  					*old_head, *new_head);
> >  	} while (unlikely(success == 0));
> > @@ -158,9 +160,11 @@ __rte_ring_move_cons_head(struct rte_ring *r,
> unsigned int is_sc,
> >  			return 0;
> >
> >  		*new_head = *old_head + n;
> > -		if (is_sc)
> > -			r->cons.head = *new_head, success = 1;
> > -		else
> > +		if (is_sc) {
> > +			r->cons.head = *new_head;
> > +			rte_smp_rmb();
> > +			success = 1;
> > +		} else
> >  			success = rte_atomic32_cmpset(&r->cons.head,
> *old_head,
> >  					*new_head);
> >  	} while (unlikely(success == 0));
> >