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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 744FFC43441 for ; Mon, 26 Nov 2018 15:34:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 44DCD20862 for ; Mon, 26 Nov 2018 15:34:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 44DCD20862 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-block-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726731AbeK0C3F (ORCPT ); Mon, 26 Nov 2018 21:29:05 -0500 Received: from mga17.intel.com ([192.55.52.151]:43195 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726248AbeK0C3F (ORCPT ); Mon, 26 Nov 2018 21:29:05 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga107.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 26 Nov 2018 07:34:37 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,282,1539673200"; d="scan'208";a="111662587" Received: from unknown (HELO localhost.localdomain) ([10.232.112.69]) by orsmga002.jf.intel.com with ESMTP; 26 Nov 2018 07:34:36 -0800 Date: Mon, 26 Nov 2018 08:31:35 -0700 From: Keith Busch To: Jens Axboe Cc: Christoph Hellwig , linux-scsi@vger.kernel.org, linux-block@vger.kernel.org, Martin Petersen , Bart Van Assche Subject: Re: [PATCHv3 2/3] scsi: Do not rely on blk-mq for double completions Message-ID: <20181126153135.GN26707@localhost.localdomain> References: <20181115175820.13391-1-keith.busch@intel.com> <20181115175820.13391-3-keith.busch@intel.com> <20181119085815.GB29626@infradead.org> <20181119151859.GB23062@localhost.localdomain> <20181121131212.GA7251@infradead.org> <913ec33d-9aff-2c48-362f-9252ed111451@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <913ec33d-9aff-2c48-362f-9252ed111451@kernel.dk> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Mon, Nov 26, 2018 at 08:32:45AM -0700, Jens Axboe wrote: > On 11/21/18 6:12 AM, Christoph Hellwig wrote: > > On Mon, Nov 19, 2018 at 08:19:00AM -0700, Keith Busch wrote: > >> On Mon, Nov 19, 2018 at 12:58:15AM -0800, Christoph Hellwig wrote: > >>>> index 5d83a162d03b..c1d5e4e36125 100644 > >>>> --- a/drivers/scsi/scsi_lib.c > >>>> +++ b/drivers/scsi/scsi_lib.c > >>>> @@ -1635,8 +1635,11 @@ static blk_status_t scsi_mq_prep_fn(struct request *req) > >>>> > >>>> static void scsi_mq_done(struct scsi_cmnd *cmd) > >>>> { > >>>> + if (unlikely(test_and_set_bit(__SCMD_COMPLETE, &cmd->flags))) > >>>> + return; > >>>> trace_scsi_dispatch_cmd_done(cmd); > >>>> - blk_mq_complete_request(cmd->request); > >>>> + if (unlikely(!blk_mq_complete_request(cmd->request))) > >>>> + clear_bit(__SCMD_COMPLETE, &cmd->flags); > >>>> } > >>> > >>> This looks a little odd to me. If we didn't complete the command > >>> someone else did. Why would we clear the bit in this case? > >> > >> It's only to go along with the fake timeout. If we don't clear the bit, > >> then then scsi timeout handler will believe it has nothing to do because > >> scsi did its required part. The block layer just pretends the LLD didn't > >> do its part, so scsi has to play along too. > > > > This just looks way to magic to me. In other word - it needs a big fat > > comment explaining the situation. > > > >>>> +#define __SCMD_COMPLETE 3 > >>>> +#define SCMD_COMPLETE (1 << __SCMD_COMPLETE) > >>> > >>> This mixing of atomic and non-atomic bitops looks rather dangerous > >>> to me. Can you add a new atomic_flags just for the completed flag, > >>> and always use the bitops on it for now? I think we can eventually > >>> kill most of the existing flags except for SCMD_TAGGED over the > >>> next merge window or two and then move that over as well. > >> > >> The only concurrent access is completion + timeout, otherwise access is > >> single-threaded. I'm using the atomic operations only where it is > >> needed. > >> > >> We implicitly clear the SCMD_COMPLETED flag along with SCMD_TAGGED in > >> scsi_init_command() too, and I didn't want to add new overhead with > >> new atomics. > > > > In general mixing access types on a single field (nevermind bit) > > is going to cause us problems further down the road sooner or later. > > > > I'd be much happier with a separate field. > > Keith, will you please respin with the separate field? Would be nice > to get this merged for 4.21. I'll send out a new version today.