> -----Original Message-----
> From: Gavin Hu [mailto:gavin...@arm.com]
> Sent: Tuesday, March 12, 2019 4:59 PM
> To: dev@dpdk.org
> Cc: n...@arm.com; gavin hu <gavin...@arm.com>; tho...@monjalon.net; Ananyev,
> Konstantin <konstantin.anan...@intel.com>;
> jer...@marvell.com; hemant.agra...@nxp.com; nipun.gu...@nxp.com;
> honnappa.nagaraha...@arm.com; i.maxim...@samsung.com;
> chao...@linux.vnet.ibm.com; sta...@dpdk.org
> Subject: [PATCH v3 1/1] ring: enforce reading the tail before reading ring
> slots
>
> From: gavin hu <gavin...@arm.com>
>
> In weak memory models, like arm64, reading the prod.tail may get
> reordered after reading 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
> 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. For SP case, the control depependency between if-statement(which
> depends on the read of r->cons.tail) and the later stores to the ring slots
> make RMB unnecessary. About the control dependency, read more at:
> https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test7.pdf
>
> This patch is adding the required read barrier to prevent reading the ring
> slots get reordered before reading prod.tail for SC case.
>
> Fixes: c9fb3c62896f ("ring: move code in a new header file")
> Cc: sta...@dpdk.org
>
> 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 | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/lib/librte_ring/rte_ring_generic.h
> b/lib/librte_ring/rte_ring_generic.h
> index ea7dbe5..953cdbb 100644
> --- a/lib/librte_ring/rte_ring_generic.h
> +++ b/lib/librte_ring/rte_ring_generic.h
> @@ -158,11 +158,14 @@ __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));
> return n;
> }
> --
Acked-by: Konstantin Ananyev <konstantin.anan...@intel.com>
> 2.7.4
