From owner-freebsd-hackers@FreeBSD.ORG Mon Sep 19 03:53:09 2011 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 3239F106566C; Mon, 19 Sep 2011 03:53:09 +0000 (UTC) (envelope-from lacombar@gmail.com) Received: from mail-wy0-f178.google.com (mail-wy0-f178.google.com [74.125.82.178]) by mx1.freebsd.org (Postfix) with ESMTP id 8A88D8FC14; Mon, 19 Sep 2011 03:53:08 +0000 (UTC) Received: by wyf23 with SMTP id 23so6023047wyf.37 for ; Sun, 18 Sep 2011 20:53:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; bh=zlv5n24+GcZz9BEp0uysYzXFADJdGCL2EQztP3GMpWY=; b=mHdVJ3WlYTGahJahN1NHcZbKuEH5pLomg9ZA4qgwhoaMAPToxyMvSipQW5EfBThEz+ fqzjnpA1lBGXb8mLJivNCUCgeco8BkKswMGv/cW2KpfO5lFyKFMCyAmmG580icwrtdDP G1ClpVaVFKmODHAXXoOyauSJSTeNe4hs0RLqM= MIME-Version: 1.0 Received: by 10.216.136.159 with SMTP id w31mr2180394wei.53.1316404387237; Sun, 18 Sep 2011 20:53:07 -0700 (PDT) Received: by 10.180.95.169 with HTTP; Sun, 18 Sep 2011 20:53:07 -0700 (PDT) In-Reply-To: References: Date: Sun, 18 Sep 2011 23:53:07 -0400 Message-ID: From: Arnaud Lacombe To: "K. Macy" Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Cc: FreeBSD Hackers Subject: Re: buf_ring(9) API precisions X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 19 Sep 2011 03:53:09 -0000 Hi, On Fri, Sep 16, 2011 at 10:41 AM, K. Macy wrote: > On Fri, Sep 16, 2011 at 3:02 AM, Arnaud Lacombe wrot= e: >> Hi, >> >> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe wr= ote: >>> Hi Kip, >>> >>> I've got a few question about the buf_ring(9) API. >>> >>> 1) what means the 'drbr_' prefix. I can guess the two last letter, 'b' >>> and 'r', for Buffer Ring, but what about 'd' and 'r' ? >>> >>> 2) in `sys/sys/buf_ring.h', you defined 'struct buf_ring' as: >>> >>> struct buf_ring { >>> =A0 =A0 =A0 =A0volatile uint32_t =A0 =A0 =A0 br_prod_head; >>> =A0 =A0 =A0 =A0volatile uint32_t =A0 =A0 =A0 br_prod_tail; >>> =A0 =A0 =A0 =A0int =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 br_prod_size= ; >>> =A0 =A0 =A0 =A0int =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 br_prod_mask= ; >>> =A0 =A0 =A0 =A0uint64_t =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0br_drops; >>> =A0 =A0 =A0 =A0uint64_t =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0br_prod_bufs; >>> =A0 =A0 =A0 =A0uint64_t =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0br_prod_bytes; >> shouldn't those 3 fields be updated atomically, especially on 32bits >> platforms ? That might pose a problem as, AFAIK, FreeBSD do not have >> MI 64bits atomics operations... > > Between the point at which br_prod_tail =3D=3D prod_head and when we > update br_prod_tail to point to prod_next we are the exclusive owners > of the fields in buf_ring. That is why we wait for any other > enqueueing threads to update br_prod_tail to point to prod_head before > continuing. > How do you enforce ordering ? I do not see anything particular forbidding the `br->br_prod_tail' to be committed first, leading other thread to believe they have access to the statistics, while the other thread has not yet committed its change. Thanks, - Arnaud > Cheers > > =A0 =A0 =A0 =A0/* > =A0 =A0 =A0 =A0 * If there are other enqueues in progress > =A0 =A0 =A0 =A0 * that preceeded us, we need to wait for them > =A0 =A0 =A0 =A0 * to complete > =A0 =A0 =A0 =A0 */ > =A0 =A0 =A0 =A0while (br->br_prod_tail !=3D prod_head) > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0cpu_spinwait(); > =A0 =A0 =A0 =A0br->br_prod_bufs++; > =A0 =A0 =A0 =A0br->br_prod_bytes +=3D nbytes; > =A0 =A0 =A0 =A0br->br_prod_tail =3D prod_next; > =A0 =A0 =A0 =A0critical_exit(); >