From owner-freebsd-arm@FreeBSD.ORG  Tue Feb 24 02:21:00 2015
Return-Path: <owner-freebsd-arm@FreeBSD.ORG>
Delivered-To: freebsd-arm@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115])
 (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id ECFC4AC6
 for <freebsd-arm@freebsd.org>; Tue, 24 Feb 2015 02:21:00 +0000 (UTC)
Received: from phabric-backend.isc.freebsd.org
 (phabric-backend.isc.freebsd.org [IPv6:2001:4f8:3:ffe0:406a:0:50:2])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client did not present a certificate)
 by mx1.freebsd.org (Postfix) with ESMTPS id C9C5579B
 for <freebsd-arm@freebsd.org>; Tue, 24 Feb 2015 02:21:00 +0000 (UTC)
Received: from phabric-backend.isc.freebsd.org
 (phabric-backend.isc.freebsd.org [127.0.1.5])
 by phabric-backend.isc.freebsd.org (8.14.9/8.14.9) with ESMTP id
 t1O2L0rk089868
 for <freebsd-arm@freebsd.org>; Tue, 24 Feb 2015 02:21:00 GMT
 (envelope-from root@phabric-backend.isc.freebsd.org)
Received: (from root@localhost)
 by phabric-backend.isc.freebsd.org (8.14.9/8.14.9/Submit) id t1O2L0g9089865;
 Tue, 24 Feb 2015 02:21:00 GMT (envelope-from root)
Date: Tue, 24 Feb 2015 02:21:00 +0000
To: freebsd-arm@freebsd.org
From: "kmacy (Kip Macy)" <phabric-noreply@FreeBSD.org>
Subject: [Differential] [Commented On] D1833: Add memory barriers to buf_ring
Message-ID: <2be0f8f73e751b59e35a2d2e802d4379@localhost.localdomain>
X-Priority: 3
Thread-Topic: D1833: Add memory barriers to buf_ring
X-Herald-Rules: none
X-Phabricator-To: <PHID-USER-6m5mo2kjikkzyd33ld4n>
X-Phabricator-To: <PHID-USER-22avn2hssqzufmcg2crf>
X-Phabricator-To: <PHID-USER-k5bbpaejfd56ik2tjj37>
X-Phabricator-To: <PHID-USER-q5lmute3rwskeizvu5gf>
X-Phabricator-Cc: <PHID-USER-ueipubl3jymguf56hiqy>
X-Phabricator-Cc: <PHID-USER-xvjulcphpj66vffzpxft>
X-Phabricator-Cc: <PHID-USER-pns6zgllnr6ze4h7ms3o>
X-Phabricator-Cc: <PHID-USER-yhwnadxnjthofqelhkdx>
X-Phabricator-Cc: <PHID-USER-wmu3dipuua4kxljjqlii>
X-Phabricator-Cc: <PHID-MLST-stsrni3uozki6w7zubvy>
In-Reply-To: <differential-rev-PHID-DREV-ntnotl4wj5wcj2eoukly-req@FreeBSD.org>
References: <differential-rev-PHID-DREV-ntnotl4wj5wcj2eoukly-req@FreeBSD.org>
Thread-Index: OGRiNDkxY2NmMjRiNTc0MjQ4YTYwNWVkNzIyIFTr4Aw=
X-Phabricator-Sent-This-Message: Yes
X-Mail-Transport-Agent: MetaMTA
X-Auto-Response-Suppress: All
X-Phabricator-Mail-Tags: <differential-comment>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="utf-8"
X-BeenThere: freebsd-arm@freebsd.org
X-Mailman-Version: 2.1.18-1
Precedence: list
List-Id: "Porting FreeBSD to ARM processors." <freebsd-arm.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-arm>,
 <mailto:freebsd-arm-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arm/>
List-Post: <mailto:freebsd-arm@freebsd.org>
List-Help: <mailto:freebsd-arm-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arm>,
 <mailto:freebsd-arm-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 24 Feb 2015 02:21:01 -0000

kmacy added a comment.

I've been spending more time looking at this than I care to admit. I don't understand why we need the load_acq_32 on prod_tail. The atomic_store_rel_int in enqueue should guarantee that the store to the ring happens before the update to prod_tail. So the consumer should _never_ see a prod_tail that is newer than the ring[prod_next] value. I suspect that something else is going on that we don't understand.

REVISION DETAIL
  https://reviews.freebsd.org/D1833

To: zbb, kmacy, rpaulo, imp
Cc: meloun-miracle-cz, onwahe-gmail-com, andrew, ian, adrian, freebsd-arm