From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751766AbdJDRS7 (ORCPT ); Wed, 4 Oct 2017 13:18:59 -0400 Received: from mail-pg0-f49.google.com ([74.125.83.49]:49391 "EHLO mail-pg0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751357AbdJDRS4 (ORCPT ); Wed, 4 Oct 2017 13:18:56 -0400 X-Google-Smtp-Source: AOwi7QDA3DWntX9R2T33qLmI7ilZutEQo71Fuczzi/VZ6DG1NPe0Vgvu7eggIKyCUPIePLern/gveQ== Subject: Re: [PATCH -v2] blk-mq: Start to fix memory ordering... To: Peter Zijlstra Cc: parri.andrea@gmail.com, linux-kernel@vger.kernel.org, tom.leiming@gmail.com, hch@lst.de, paulmck@linux.vnet.ibm.com, will.deacon@arm.com, boqun.feng@gmail.com, stern@rowland.harvard.edu References: <20170906080022.sotl4pvihs2nvg4m@hirez.programming.kicks-ass.net> From: Jens Axboe Message-ID: <26c964cd-4ad8-7bbb-6fc9-3c5dafa2117d@kernel.dk> Date: Wed, 4 Oct 2017 11:18:53 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <20170906080022.sotl4pvihs2nvg4m@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/06/2017 02:00 AM, Peter Zijlstra wrote: > > Attempt to untangle the ordering in blk-mq. The patch introducing the > single smp_mb__before_atomic() is obviously broken in that it doesn't > clearly specify a pairing barrier and an obtained guarantee. > > The comment is further misleading in that it hints that the > deadline store and the COMPLETE store also need to be ordered, but > AFAICT there is no such dependency. However what does appear to be > important is the clear happening _after_ the store, and that worked by > pure accident. > > This clarifies blk_mq_start_request() -- we should not get there with > STARTING set -- this simplifies the code and makes the barrier usage > sane (the old code could be read to allow not having _any_ atomic after > the barrier, in which case the barrier hasn't got anything to order). We > then also introduce the missing pairing barrier for it. > > Also down-grade the barrier to smp_wmb(), this is cheaper for > PowerPC/ARM and doesn't cost anything extra on x86. > > And it documents the STARTING vs COMPLETE ordering. Although I've not > been entirely successful in reverse engineering the blk-mq state > machine so there might still be more funnies around timeout vs > requeue. > > If I got anything wrong, feel free to educate me by adding comments to > clarify things ;-) Sorry for the belated response on this, I spent some time and looked over everything. Looks solid to me. I'll queue this up for some testing, and also add a compile check to prevent us violating the need to have STARTED and COMPLETED be in the same byte of storage. -- Jens Axboe