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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 242B4C43441 for ; Wed, 21 Nov 2018 13:12:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D7DE5214C4 for ; Wed, 21 Nov 2018 13:12:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="B1uNtGnx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D7DE5214C4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org 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 S1730881AbeKUXqc (ORCPT ); Wed, 21 Nov 2018 18:46:32 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:54122 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726016AbeKUXqc (ORCPT ); Wed, 21 Nov 2018 18:46:32 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=/2IGolipt7iMZwOSiSPhHOrxkeULUg7KkhIguirXgN8=; b=B1uNtGnx9z90HBQ/eUceZq5Dk ihEF5R+Es8tOpnof3L5CaYBYHh3NOQarXHEZ3gc+WgyMXGM2MmhW1YPquTDvy8lgkuZjnMOuUikYz 3tb3jBR6JL0w2eQufZRGWUZOwYYaqm3LnTTRt19MJfldBkAuCav6OmCJt0Q9jF5yOvHxEPtyFOis5 dNj8cQDJbViyIGJjRtR1aHpGlC15tb5zzNTJUCtEoyXbGI0/VaYIodGtdg5cpn5DeZHVvhjM0Tm9l DOqKuUsPIZVrJYEADulFAfrrYO1H/j+yfWOsDPxj+26tjol4uLfZasXhDt56F90D2Q+wgWeUqp3aH v8pip6r0Q==; Received: from hch by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1gPSIe-0003bH-72; Wed, 21 Nov 2018 13:12:12 +0000 Date: Wed, 21 Nov 2018 05:12:12 -0800 From: Christoph Hellwig To: Keith Busch Cc: Christoph Hellwig , linux-scsi@vger.kernel.org, linux-block@vger.kernel.org, Jens Axboe , Martin Petersen , Bart Van Assche Subject: Re: [PATCHv3 2/3] scsi: Do not rely on blk-mq for double completions Message-ID: <20181121131212.GA7251@infradead.org> References: <20181115175820.13391-1-keith.busch@intel.com> <20181115175820.13391-3-keith.busch@intel.com> <20181119085815.GB29626@infradead.org> <20181119151859.GB23062@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181119151859.GB23062@localhost.localdomain> User-Agent: Mutt/1.9.2 (2017-12-15) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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.