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=-6.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 6DDE2C433DF for ; Fri, 19 Jun 2020 10:12:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 45FA72073E for ; Fri, 19 Jun 2020 10:12:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="SJvnA0F9" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732283AbgFSKME (ORCPT ); Fri, 19 Jun 2020 06:12:04 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:34881 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1732225AbgFSKMB (ORCPT ); Fri, 19 Jun 2020 06:12:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592561520; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=WvnaRcnfDA3IL8aXR7gMxiymCTyL1NKRb2G8dpseDfE=; b=SJvnA0F9oXfwPoLv4ZQULdbD8kg2+RSLVr8CpQWhQ4Yefp5yDsYQZfz5LA/+v7oZn/BkCC HuFVNWHoMoYovNYB7g97qcyIvTD5VO+Vk0e/00DFcvs2WTAZkrqR2/5UTjRSEJRVyXjSnf SYRtIby6MhCnpfrPd7vfMBzfn+ljHS4= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-282-KDPzoXx4PqmwmkObSWeS8A-1; Fri, 19 Jun 2020 06:11:58 -0400 X-MC-Unique: KDPzoXx4PqmwmkObSWeS8A-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 618AB8015CB for ; Fri, 19 Jun 2020 10:11:57 +0000 (UTC) Received: from T590 (ovpn-12-44.pek2.redhat.com [10.72.12.44]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 59F6F5BAC1; Fri, 19 Jun 2020 10:11:46 +0000 (UTC) Date: Fri, 19 Jun 2020 18:11:42 +0800 From: Ming Lei To: Mike Snitzer Cc: dm-devel@redhat.com, linux-block@vger.kernel.org Subject: Re: dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue() Message-ID: <20200619101142.GA339442@T590> References: <20200619084214.337449-1-ming.lei@redhat.com> <20200619094250.GA18410@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200619094250.GA18410@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi Mike, On Fri, Jun 19, 2020 at 05:42:50AM -0400, Mike Snitzer wrote: > Hi Ming, > > Thanks for the patch! But I'm having a hard time understanding what > you've written in the patch header, > > On Fri, Jun 19 2020 at 4:42am -0400, > Ming Lei wrote: > > > dm-rq won't stop queue, meantime blk-mq won't stop one queue too, so > > remove the check. > > It'd be helpful if you could unpack this with more detail before going on > to explain why using blk_queue_quiesced, despite dm-rq using > blk_mq_queue_stopped, would also be ineffective. > > SO: > > > dm-rq won't stop queue > > 1) why won't dm-rq stop the queue? Do you mean it won't reliably > _always_ stop the queue because of the blk_mq_queue_stopped() check? device mapper doesn't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues. > > > meantime blk-mq won't stop one queue too, so remove the check. > > 2) Meaning?: blk_mq_queue_stopped() will return true even if only one hw > queue is stopped, given blk-mq must stop all hw queues a positive return > from this blk_mq_queue_stopped() check is incorrectly assuming it meanss > all hw queues are stopped. blk-mq won't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues for dm-rq's queue too, so dm-rq's hw queue won't be stopped. BTW blk_mq_stop_hw_queue or blk_mq_stop_hw_queues are supposed to be used for throttling queue. > > > dm_stop_queue() actually tries to quiesce hw queues via blk_mq_quiesce_queue(), > > we can't check via blk_queue_quiesced for avoiding unnecessary queue > > quiesce because the flag is set before synchronize_rcu() and dm_stop_queue > > may be called when synchronize_rcu from another blk_mq_quiesce_queue is > > in-progress. > > But I'm left with questions/confusion on this too: > > 1) you mention blk_queue_quiesced instead of blk_mq_queue_stopped, so I > assume you mean that: not only is blk_mq_queue_stopped() > ineffective, blk_queue_quiesced() would be too? blk_mq_queue_stopped isn't necessary because dm-rq's hw queue won't be stopped by anyone, meantime replacing it with blk_queue_quiesced() is wrong. > > 2) the race you detail (with competing blk_mq_quiesce_queue) relative to > synchronize_rcu() and testing "the flag" is very detailed yet vague. If two code paths are calling dm_stop_queue() at the same time, one path may return immediately and it is wrong, sine synchronize_rcu() from another path may not be done. > > Anyway, once we get this heaader cleaned up a bit more I'll be happy to > get this staged as a stable@ fix for 5.8 inclusion ASAP. This patch isn't a fix, and it shouldn't be related with rhel8's issue. Thanks, Ming > > Thanks! > Mike > > > > > Cc: linux-block@vger.kernel.org > > Signed-off-by: Ming Lei > > --- > > drivers/md/dm-rq.c | 3 --- > > 1 file changed, 3 deletions(-) > > > > diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c > > index f60c02512121..ed4d5ea66ccc 100644 > > --- a/drivers/md/dm-rq.c > > +++ b/drivers/md/dm-rq.c > > @@ -70,9 +70,6 @@ void dm_start_queue(struct request_queue *q) > > > > void dm_stop_queue(struct request_queue *q) > > { > > - if (blk_mq_queue_stopped(q)) > > - return; > > - > > blk_mq_quiesce_queue(q); > > } > > > > -- > > 2.25.2 > > -- Ming