From owner-freebsd-hackers@FreeBSD.ORG Mon Sep 19 12:46:38 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 967B5106566B for ; Mon, 19 Sep 2011 12:46:38 +0000 (UTC) (envelope-from kmacybsd@gmail.com) Received: from mail-yx0-f182.google.com (mail-yx0-f182.google.com [209.85.213.182]) by mx1.freebsd.org (Postfix) with ESMTP id 532D08FC0C for ; Mon, 19 Sep 2011 12:46:38 +0000 (UTC) Received: by yxk36 with SMTP id 36so5031248yxk.13 for ; Mon, 19 Sep 2011 05:46:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:cc:content-type :content-transfer-encoding; bh=l8meStHq+UiLrd2LUzsJjxETUl/EWKoxkkLakykw1hk=; b=AR3wGPQP+maVLgUYxdzfGhP2bbJnNx0YVhjsajaxRUEML92jN9vpQQYVbV3mpin1AI lgJ3RU8Qy9IkeHyAaMY0a49SfjPMJGl7z4/9IHqWNrdUjVBFRVt8L74+AVMgJutfpuf0 5A9MUbknFrpsS1eXLbDvSkXkSULCqxtjpqugQ= MIME-Version: 1.0 Received: by 10.52.96.166 with SMTP id dt6mr2145925vdb.345.1316436397637; Mon, 19 Sep 2011 05:46:37 -0700 (PDT) Sender: kmacybsd@gmail.com Received: by 10.52.113.202 with HTTP; Mon, 19 Sep 2011 05:46:37 -0700 (PDT) In-Reply-To: References: Date: Mon, 19 Sep 2011 14:46:37 +0200 X-Google-Sender-Auth: C0fLcIppiAzxhc-Y_MUV96E2B1Q Message-ID: From: "K. Macy" To: Arnaud Lacombe 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 12:46:38 -0000 If the value lags next by one then it is ours. This rule applies to all callers so the rule holds consistently. On Mon, Sep 19, 2011 at 5:53 AM, Arnaud Lacombe wrote: > Hi, > > On Fri, Sep 16, 2011 at 10:41 AM, K. Macy wrote: >> On Fri, Sep 16, 2011 at 3:02 AM, Arnaud Lacombe wro= te: >>> Hi, >>> >>> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe w= rote: >>>> 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_siz= e; >>>> =A0 =A0 =A0 =A0int =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 br_prod_mas= k; >>>> =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, > =A0- 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(); >> >