From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 35C1EC2BB1D for ; Mon, 16 Mar 2020 18:44:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0D606206E2 for ; Mon, 16 Mar 2020 18:44:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584384271; bh=6LTMC3susUR3tK6Jl9RDhS/M/VCl8aBhMASIoUftteM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=TgPXrRmElqM8FNA93Lo5tsptU0o4LdrXz0OPLGgOZy9RQvMhMqDkyjY/IyT72uaB7 +WDATSv4mEXqA8gE2sI7ZpxTm3StN99e+GWOzTGX2LlHTo8bYibLVhYMeVVzL6fKZe o+iIqhKuX7xtOCll1G4yC0Ao3klXfZRBTkEjWIvE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732425AbgCPSoa (ORCPT ); Mon, 16 Mar 2020 14:44:30 -0400 Received: from mail.kernel.org ([198.145.29.99]:35454 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732392AbgCPSo3 (ORCPT ); Mon, 16 Mar 2020 14:44:29 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1F2C220674; Mon, 16 Mar 2020 18:44:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584384268; bh=6LTMC3susUR3tK6Jl9RDhS/M/VCl8aBhMASIoUftteM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=qqaEBB64GEQga3hzq4Ztkwo13alDADzVf7+6cLFAcnT20rD4I36LnuPcjLAzoA8VM f7GMQDq0UWKAI5mvtSjMUEd8YT6mJXBf3roVFeHGlpej6+LxHtCItDzo3YndcNBAPY TKypfIa1736bbGgjicRLsPenIIZ/fk0FpVmmPGLk= Date: Mon, 16 Mar 2020 18:44:23 +0000 From: Will Deacon To: =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= Cc: Daniel Borkmann , John Fastabend , Netdev , Alexei Starovoitov , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , bpf , "Karlsson, Magnus" , Jonathan Lemon , mark.rutland@arm.com Subject: Re: [PATCH bpf-next] xsk: update rings for load-acquire/store-release semantics Message-ID: <20200316184423.GA14143@willie-the-truck> References: <20200120092149.13775-1-bjorn.topel@gmail.com> <28b2b6ba-7f43-6cab-9b3a-174fc71d5a62@iogearbox.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, Jan 21, 2020 at 12:50:23PM +0100, Björn Töpel wrote: > On Tue, 21 Jan 2020 at 00:51, Daniel Borkmann wrote: > > > > On 1/20/20 10:21 AM, Björn Töpel wrote: > > > From: Björn Töpel > > > > > > Currently, the AF_XDP rings uses fences for the kernel-side > > > produce/consume functions. By updating rings for > > > load-acquire/store-release semantics, the full barrier (smp_mb()) on > > > the consumer side can be replaced. > > > > > > Signed-off-by: Björn Töpel > > > > If I'm not missing something from the ring update scheme, don't you also need > > to adapt to STORE.rel ->producer with matching barrier in tools/lib/bpf/xsk.h ? > > > > Daniel/John, > > Hmm, I was under the impression that *wasn't* the case. Quoting > memory-barriers.txt: > > --8<-- > When dealing with CPU-CPU interactions, certain types of memory > barrier should always be paired. A lack of appropriate pairing is > almost certainly an error. > > General barriers pair with each other, though they also pair with most > other types of barriers, albeit without multicopy atomicity. An > acquire barrier pairs with a release barrier, but both may also pair > with other barriers, including of course general barriers. A write > barrier pairs with a data dependency barrier, a control dependency, an > acquire barrier, a release barrier, a read barrier, or a general > barrier. Similarly a read barrier, control dependency, or a data > dependency barrier pairs with a write barrier, an acquire barrier, a > release barrier, or a general barrier: > -->8-- The key part here is "albeit without multicopy atomicity". I don't think you care about that at all for these rings as you're very clearly passing a message from the producer side to the consumer side in a point-to-point like manner, so I think you're ok to change the kernel independently from userspace (but I would still recommend updating both eventually). The only thing you might run into is if anybody is relying on the smp_mb() in the consumer to order other unrelated stuff either side of the consume operation (or even another consume operation to a different ring!), but it looks like you can't rely on that in the xsk queue implementation anyway because you cache the global state and so the barriers are conditional. Will