Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752374AbbKKKmV (ORCPT ); Wed, 11 Nov 2015 05:42:21 -0500 Received: from www62.your-server.de ([213.133.104.62]:54765 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751766AbbKKKmT (ORCPT ); Wed, 11 Nov 2015 05:42:19 -0500 Message-ID: <56431B83.5060500@iogearbox.net> Date: Wed, 11 Nov 2015 11:42:11 +0100 From: Daniel Borkmann User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: Will Deacon , Arnd Bergmann CC: linaro-kernel@lists.linaro.org, Z Lim , Alexei Starovoitov , "Shi, Yang" , Eric Dumazet , Catalin Marinas , Alexei Starovoitov , LKML , "linux-arm-kernel@lists.infradead.org" , Network Development , Xi Wang Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction References: <1447195301-16757-1-git-send-email-yang.shi@linaro.org> <20151111004208.GA47378@ast-mbp.thefacebook.com> <4902833.k8y8bz0YLV@wuerfel> <20151111102406.GB9562@arm.com> In-Reply-To: <20151111102406.GB9562@arm.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1801 Lines: 41 On 11/11/2015 11:24 AM, Will Deacon wrote: > On Wed, Nov 11, 2015 at 09:49:48AM +0100, Arnd Bergmann wrote: >> On Tuesday 10 November 2015 18:52:45 Z Lim wrote: >>> On Tue, Nov 10, 2015 at 4:42 PM, Alexei Starovoitov >>> wrote: >>>> On Tue, Nov 10, 2015 at 04:26:02PM -0800, Shi, Yang wrote: >>>>> On 11/10/2015 4:08 PM, Eric Dumazet wrote: >>>>>> On Tue, 2015-11-10 at 14:41 -0800, Yang Shi wrote: >>>>>>> aarch64 doesn't have native support for XADD instruction, implement it by >>>>>>> the below instruction sequence: >>> >>> aarch64 supports atomic add in ARMv8.1. >>> For ARMv8(.0), please consider using LDXR/STXR sequence. >> >> Is it worth optimizing for the 8.1 case? It would add a bit of complexity >> to make the code depend on the CPU feature, but it's certainly doable. > > What's the atomicity required for? Put another way, what are we racing > with (I thought bpf was single-threaded)? Do we need to worry about > memory barriers? > > Apologies if these are stupid questions, but all I could find was > samples/bpf/sock_example.c and it didn't help much :( The equivalent code more readable in restricted C syntax (that can be compiled by llvm) can be found in samples/bpf/sockex1_kern.c. So the built-in __sync_fetch_and_add() will be translated into a BPF_XADD insn variant. What you can race against is that an eBPF map can be _shared_ by multiple eBPF programs that are attached somewhere in the system, and they could all update a particular entry/counter from the map at the same time. Best, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/