Re: [dpdk-dev] [PATCH v3 1/1] ring: enforce reading the tail before reading ring slots

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Gavin Hu [mailto:gavin.hu@arm.com]
> Sent: Tuesday, March 12, 2019 4:59 PM
> To: dev@dpdk.org
> Cc: nd@arm.com; gavin hu <gavin.hu@arm.com>; thomas@monjalon.net; Ananyev, Konstantin <konstantin.ananyev@intel.com>;
> jerinj@marvell.com; hemant.agrawal@nxp.com; nipun.gupta@nxp.com; Honnappa.Nagarahalli@arm.com; i.maximets@samsung.com;
> chaozhu@linux.vnet.ibm.com; stable@dpdk.org
> Subject: [PATCH v3 1/1] ring: enforce reading the tail before reading ring slots
> 
> From: gavin hu <gavin.hu@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: stable@dpdk.org
> 
> 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 | 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.ananyev@intel.com>

> 2.7.4