From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Monjalon <thomas@monjalon.net>
Subject: Re: [PATCH v3 1/1] ring: enforce reading the tail before
	reading ring slots
Date: Thu, 28 Mar 2019 01:21:55 +0100
Message-ID: <1963883.zIr16fWVUr@xps>
References: <1551841661-42892-1-git-send-email-gavin.hu@arm.com>
 <1552409933-45684-2-git-send-email-gavin.hu@arm.com>
 <2601191342CEEE43887BDE71AB977258013655C014@irsmsx105.ger.corp.intel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7Bit
Cc: dev@dpdk.org, "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
 "nd@arm.com" <nd@arm.com>, "jerinj@marvell.com" <jerinj@marvell.com>,
 "hemant.agrawal@nxp.com" <hemant.agrawal@nxp.com>,
 "nipun.gupta@nxp.com" <nipun.gupta@nxp.com>,
 "Honnappa.Nagarahalli@arm.com" <Honnappa.Nagarahalli@arm.com>,
 "i.maximets@samsung.com" <i.maximets@samsung.com>,
 "chaozhu@linux.vnet.ibm.com" <chaozhu@linux.vnet.ibm.com>,
 "stable@dpdk.org" <stable@dpdk.org>
To: Gavin Hu <gavin.hu@arm.com>
Return-path: <dev-bounces@dpdk.org>
In-Reply-To: <2601191342CEEE43887BDE71AB977258013655C014@irsmsx105.ger.corp.intel.com>
List-Id: DPDK patches and discussions <dev.dpdk.org>
List-Unsubscribe: <https://mails.dpdk.org/options/dev>,
 <mailto:dev-request@dpdk.org?subject=unsubscribe>
List-Archive: <http://mails.dpdk.org/archives/dev/>
List-Post: <mailto:dev@dpdk.org>
List-Help: <mailto:dev-request@dpdk.org?subject=help>
List-Subscribe: <https://mails.dpdk.org/listinfo/dev>,
 <mailto:dev-request@dpdk.org?subject=subscribe>
Errors-To: dev-bounces@dpdk.org
Sender: "dev" <dev-bounces@dpdk.org>

> > 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>
> 
> Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>

Applied, thanks