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=-16.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 B1D25C4338F for ; Sat, 21 Aug 2021 00:31:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9101261042 for ; Sat, 21 Aug 2021 00:31:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232139AbhHUAc1 (ORCPT ); Fri, 20 Aug 2021 20:32:27 -0400 Received: from mail.kernel.org ([198.145.29.99]:36668 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229783AbhHUAc0 (ORCPT ); Fri, 20 Aug 2021 20:32:26 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 1BC0861175 for ; Sat, 21 Aug 2021 00:31:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1629505900; bh=U5s7pwqFQejZnbF2637G7Qe18H+Si0LOHqFwd5P6O20=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=vFEGTodj7JtYEVcu1Za1Hqc10dW8EZvDiMaa/8ceET9DdgvfzK835+aO4FT9uHF+c 8gFi9iGcoNLJj7BdjVmJBvuYAhwg/2lf0b8cEjqXELWpylL6Oc9PjP+HvvE61GNK9c DIKrNGfNzRcMDbC78q4IQc7grs2QFFxTcVqPQgV5Vm0sF8oK13uS3BQ13YHns2jPAS 04mrHS8N0aVa325WRM5TGm1R+N1YyBSAL3fyDXKVHPqn5dTD9AfLap3C0UEThwyfG5 sLFaQCf3ZmUVSZQMCWjHCuuau7FzL18ubprWpTg4wuNy2ufaf+8bJ6eJa339I0yEJr t5M0f69JIzEJA== Received: by mail-lj1-f174.google.com with SMTP id q21so19876968ljj.6 for ; Fri, 20 Aug 2021 17:31:39 -0700 (PDT) X-Gm-Message-State: AOAM53268WIaNcAyyCESEQUVfmB2/dwPl2PcxYdHojNV2KVZ6KaCcpSh Yu79EkByYckXjVCKOBd7Y3hdTUxp+0LB8+KLKiY= X-Google-Smtp-Source: ABdhPJwAF9Lu3mWp7KbiAZO/3fvFUQ+/CJouh/tMgFdP1KWR6fhf/gOpLaMovLwFw2n55B2wWE85LFBZ+R290cghvEI= X-Received: by 2002:a2e:9953:: with SMTP id r19mr17896098ljj.270.1629505898512; Fri, 20 Aug 2021 17:31:38 -0700 (PDT) MIME-Version: 1.0 References: <1629266268-3624-1-git-send-email-xni@redhat.com> In-Reply-To: <1629266268-3624-1-git-send-email-xni@redhat.com> From: Song Liu Date: Fri, 20 Aug 2021 17:31:27 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2] md/raid10: Remove rcu_dereference when it doesn't need rcu lock to protect To: Xiao Ni Cc: Nigel Croxon , linux-raid , Guoqing Jiang Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-raid@vger.kernel.org On Tue, Aug 17, 2021 at 10:58 PM Xiao Ni wrote: > > One warning message is triggered like this: > [ 695.110751] ============================= > [ 695.131439] WARNING: suspicious RCU usage > [ 695.151389] 4.18.0-319.el8.x86_64+debug #1 Not tainted > [ 695.174413] ----------------------------- > [ 695.192603] drivers/md/raid10.c:1776 suspicious > rcu_dereference_check() usage! > [ 695.225107] other info that might help us debug this: > [ 695.260940] rcu_scheduler_active = 2, debug_locks = 1 > [ 695.290157] no locks held by mkfs.xfs/10186. > > In the first loop of function raid10_handle_discard. It already > determines which disk need to handle discard request and add the > rdev reference count rdev->nr_pending. So the conf->mirrors will > not change until all bios come back from underlayer disks. It > doesn't need to use rcu_dereference to get rdev. > > Fixes: d30588b2731f ('md/raid10: improve raid10 discard request') > Signed-off-by: Xiao Ni > Acked-by: Guoqing Jiang Applied to md-fixes. Thanks! Song > --- > V2: Fix comment style problem > > drivers/md/raid10.c | 14 ++++++++++---- > 1 file changed, 10 insertions(+), 4 deletions(-) > > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c > index 16977e8..d5d9233 100644 > --- a/drivers/md/raid10.c > +++ b/drivers/md/raid10.c > @@ -1712,6 +1712,11 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio) > } else > r10_bio->master_bio = (struct bio *)first_r10bio; > > + /* > + * first select target devices under rcu_lock and > + * inc refcount on their rdev. Record them by setting > + * bios[x] to bio > + */ > rcu_read_lock(); > for (disk = 0; disk < geo->raid_disks; disk++) { > struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev); > @@ -1743,9 +1748,6 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio) > for (disk = 0; disk < geo->raid_disks; disk++) { > sector_t dev_start, dev_end; > struct bio *mbio, *rbio = NULL; > - struct md_rdev *rdev = rcu_dereference(conf->mirrors[disk].rdev); > - struct md_rdev *rrdev = rcu_dereference( > - conf->mirrors[disk].replacement); > > /* > * Now start to calculate the start and end address for each disk. > @@ -1775,9 +1777,12 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio) > > /* > * It only handles discard bio which size is >= stripe size, so > - * dev_end > dev_start all the time > + * dev_end > dev_start all the time. > + * It doesn't need to use rcu lock to get rdev here. We already > + * add rdev->nr_pending in the first loop. > */ > if (r10_bio->devs[disk].bio) { > + struct md_rdev *rdev = conf->mirrors[disk].rdev; > mbio = bio_clone_fast(bio, GFP_NOIO, &mddev->bio_set); > mbio->bi_end_io = raid10_end_discard_request; > mbio->bi_private = r10_bio; > @@ -1790,6 +1795,7 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio) > bio_endio(mbio); > } > if (r10_bio->devs[disk].repl_bio) { > + struct md_rdev *rrdev = conf->mirrors[disk].replacement; > rbio = bio_clone_fast(bio, GFP_NOIO, &mddev->bio_set); > rbio->bi_end_io = raid10_end_discard_request; > rbio->bi_private = r10_bio; > -- > 2.7.5 >