> -----Original Message-----
> From: Ilya Maximets <i.maxim...@samsung.com>
> Sent: Thursday, March 7, 2019 4:52 PM
> To: Gavin Hu (Arm Technology China) <gavin...@arm.com>;
> dev@dpdk.org
> Cc: nd <n...@arm.com>; tho...@monjalon.net; jer...@marvell.com;
> hemant.agra...@nxp.com; nipun.gu...@nxp.com; Honnappa Nagarahalli
> <honnappa.nagaraha...@arm.com>; olivier.m...@6wind.com
> Subject: Re: [PATCH v2] ring: enforce reading the tails before ring
> operations
>
> On 07.03.2019 9:45, 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.
> >
> > For MP (and MC) case, rte_atomic32_cmpset() already provides the
> required
> > ordering. This patch is to prevent reading and writing the ring slots get
> > reordered before reading {prod,cons}.tail for SP (and SC) case.
>
> Read barrier rte_smp_rmb() is OK to prevent reading the ring get
> reordered
> before reading the tail. However, to prevent *writing* the ring get
> reordered
> *before reading* the tail you need a full memory barrier, i.e.
> rte_smp_mb().
ISHLD(rte_smp_rmb is DMB(ishld) orders LD/LD and LD/ST, while WMB(ST Option)
orders ST/ST.
For more details, please refer to: Table B2-1 Encoding of the DMB and DSB
<option> parameter in
https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile
>
> >
> > Signed-off-by: gavin hu <gavin...@arm.com>
> > Reviewed-by: Ola Liljedahl <ola.liljed...@arm.com>
> > Tested-by: Nipun Gupta <nipun.gu...@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));
> >
