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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2D7AAC433FE for ; Tue, 2 Nov 2021 14:41:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1089961100 for ; Tue, 2 Nov 2021 14:41:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231266AbhKBOoB (ORCPT ); Tue, 2 Nov 2021 10:44:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44648 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231157AbhKBOoB (ORCPT ); Tue, 2 Nov 2021 10:44:01 -0400 Received: from mail-il1-x12e.google.com (mail-il1-x12e.google.com [IPv6:2607:f8b0:4864:20::12e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3E82CC061767 for ; Tue, 2 Nov 2021 07:41:26 -0700 (PDT) Received: by mail-il1-x12e.google.com with SMTP id i9so628207ilu.8 for ; Tue, 02 Nov 2021 07:41:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20210112.gappssmtp.com; s=20210112; h=subject:from:to:cc:references:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=/U1hxiNkFJx/rX8ebEMVcjsSUpOAIM0zNBm4rpYu4HM=; b=IljKTMzSgAJEYT1jyT98fBqHsBPcFKhwLtkUiv/LU0vLDrJg2oXJTxrto9kXJOaigc 5ovN/u8TlaLGxIdx0vDI/8U1LNsakFfkAtSKGn5lw056gKFdHLZda26jHFY35OyITD8r qUx8RkOCLcNxbL5El4iDSov1/4VuX/rd4i2fSHA3KtiuwoFTCRngF73YM3aLI6pe62nL EM7yVjCMw/qHp9bdcL1SBtkbfADUE9q/kCetuYMpIhw/6j1pvlGCUCMFe1jMpGtO48MN iZT91iFNJs61B0PpBPj/VhGjjTdoUv649xYRSitXeBCp5xRKMGwQgFSJbUPdkrszHqGd Ix+Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:subject:from:to:cc:references:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=/U1hxiNkFJx/rX8ebEMVcjsSUpOAIM0zNBm4rpYu4HM=; b=UN6P24pNLSv42TrLPQ9cPorsgecxw7ekD3WT8VQqVg7S1+fl27uSS/ypoGFRplbR2m EtIX0XSCFgKY1qJGYs+2rMkrkzN8PKvjm9yIttHp94mrPOMwNsAZKJ0JLO2Yh9c0dNsm u9lFpgxQ+1hjZjfWFtLbOFQooVQdXKprOam4FYuMGpZD24rxJ5ILTb7EPYB9YPi74R/3 RGmmd/qndypry2oUuJ1KsLgrZ7/1W6uYsd5Ewm3SsA9TKFAQrt/hR9vAklJzoalq9IJ7 lR1KCQi88gsMTXiyrn9zae0J1dNGOq/hVI/7psL2B9NlmovQgQiuXFva1ikA+E3ouAve zmbQ== X-Gm-Message-State: AOAM5339eO/r0lx++ft5+2jhrs3CLznErn+Evp/pdmmpQJVSxxxgC2en OvJiJ5JyZeJuL06VMGtUeeDXcQ== X-Google-Smtp-Source: ABdhPJyE7d27RzQeHYy26nP1Rz61cbOHW9AcygMT2PQRDL+okwbxgWS+4oXgVErV4Jl+LmhbSUrbLQ== X-Received: by 2002:a05:6e02:1bcb:: with SMTP id x11mr20970995ilv.94.1635864085483; Tue, 02 Nov 2021 07:41:25 -0700 (PDT) Received: from [192.168.1.30] ([207.135.234.126]) by smtp.gmail.com with ESMTPSA id s3sm7147044ilv.61.2021.11.02.07.41.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Nov 2021 07:41:24 -0700 (PDT) Subject: Re: [PATCH 2/3] scsi: make sure that request queue queiesce and unquiesce balanced From: Jens Axboe To: James Bottomley , Ming Lei Cc: Yi Zhang , linux-block@vger.kernel.org, "Martin K . Petersen" , linux-scsi@vger.kernel.org, Mike Snitzer , dm-devel@redhat.com References: <20211021145918.2691762-1-ming.lei@redhat.com> <20211021145918.2691762-3-ming.lei@redhat.com> <10c279f54ed0b24cb1ac0861f9a407e6b64f64da.camel@HansenPartnership.com> <8cbc1be6-15a5-ed34-53f1-081a05025d34@kernel.dk> <1ab71603-0104-2071-02c9-d6c22e3aa275@kernel.dk> Message-ID: <042056b5-6fea-1bcf-bfae-274f23e9e5c5@kernel.dk> Date: Tue, 2 Nov 2021 08:41:23 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <1ab71603-0104-2071-02c9-d6c22e3aa275@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 11/2/21 8:36 AM, Jens Axboe wrote: > On 11/2/21 8:33 AM, James Bottomley wrote: >> On Tue, 2021-11-02 at 06:59 -0600, Jens Axboe wrote: >>> On 11/1/21 7:43 PM, James Bottomley wrote: >>>> On Thu, 2021-10-21 at 22:59 +0800, Ming Lei wrote: >>>>> For fixing queue quiesce race between driver and block >>>>> layer(elevator switch, update nr_requests, ...), we need to >>>>> support concurrent quiesce and unquiesce, which requires the two >>>>> call balanced. >>>>> >>>>> It isn't easy to audit that in all scsi drivers, especially the >>>>> two may be called from different contexts, so do it in scsi core >>>>> with one per-device bit flag & global spinlock, basically zero >>>>> cost since request queue quiesce is seldom triggered. >>>>> >>>>> Reported-by: Yi Zhang >>>>> Fixes: e70feb8b3e68 ("blk-mq: support concurrent queue >>>>> quiesce/unquiesce") >>>>> Signed-off-by: Ming Lei >>>>> --- >>>>> drivers/scsi/scsi_lib.c | 45 ++++++++++++++++++++++++++++++ >>>>> ---- >>>>> ---- >>>>> include/scsi/scsi_device.h | 1 + >>>>> 2 files changed, 37 insertions(+), 9 deletions(-) >>>>> >>>>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c >>>>> index 51fcd46be265..414f4daf8005 100644 >>>>> --- a/drivers/scsi/scsi_lib.c >>>>> +++ b/drivers/scsi/scsi_lib.c >>>>> @@ -2638,6 +2638,40 @@ static int >>>>> __scsi_internal_device_block_nowait(struct scsi_device *sdev) >>>>> return 0; >>>>> } >>>>> >>>>> +static DEFINE_SPINLOCK(sdev_queue_stop_lock); >>>>> + >>>>> +void scsi_start_queue(struct scsi_device *sdev) >>>>> +{ >>>>> + bool need_start; >>>>> + unsigned long flags; >>>>> + >>>>> + spin_lock_irqsave(&sdev_queue_stop_lock, flags); >>>>> + need_start = sdev->queue_stopped; >>>>> + sdev->queue_stopped = 0; >>>>> + spin_unlock_irqrestore(&sdev_queue_stop_lock, flags); >>>>> + >>>>> + if (need_start) >>>>> + blk_mq_unquiesce_queue(sdev->request_queue); >>>> >>>> Well, this is a classic atomic pattern: >>>> >>>> if (cmpxchg(&sdev->queue_stopped, 1, 0)) >>>> blk_mq_unquiesce_queue(sdev->request_queue); >>>> >>>> The reason to do it with atomics rather than spinlocks is >>>> >>>> 1. no need to disable interrupts: atomics are locked >>>> 2. faster because a spinlock takes an exclusive line every time >>>> but the >>>> read to check the value can be in shared mode in cmpxchg >>>> 3. it's just shorter and better code. >>>> >>>> The only minor downside is queue_stopped now needs to be a u32. >>> >>> Are you fine with the change as-is, or do you want it redone? I >>> can drop the SCSI parts and just queue up the dm fix. Personally >>> I think it'd be better to get it fixed upfront. >> >> Well, given the path isn't hot, I don't really care. However, what I >> don't want is to have to continually bat back patches from the make >> work code churners trying to update this code for being the wrong >> pattern. I think at the very least it needs a comment saying why we >> chose a suboptimal pattern to try to forestall this. > > Right, with a comment it's probably better. And as you said, since it's > not a hot path, don't think we'd be revisiting it anyway. > > I'll amend the patch with a comment. I started adding the comment and took another look at this, and that made me change my mind. We really should make this a cmpxcgh, it's not even using a device lock here. I've dropped the two SCSI patches for now, Ming can you resend? If James agrees, I really think queue_stopped should just have the type changed and the patch redone with that using cmpxcgh(). -- Jens Axboe