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=-8.4 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,USER_AGENT_SANE_1 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 19C5CC433DF for ; Fri, 19 Jun 2020 09:43:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E6D7C2080C for ; Fri, 19 Jun 2020 09:43:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ByT/jP+w" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731501AbgFSJnE (ORCPT ); Fri, 19 Jun 2020 05:43:04 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:49611 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731861AbgFSJnE (ORCPT ); Fri, 19 Jun 2020 05:43:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592559783; 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=Usgy37+XpMwpp9KaBSHE3Q3qWZPdIgsfjefYq+Scx1E=; b=ByT/jP+wdEdg86hUKpnlRwGur3LMhXRcvIH1lwRGgBL96lHTUl+nnpDTTwNqU9dvHrDBHB xfk3glNw3jNIxFTHzRi2/iw5kDTQliL9ArS2HiSsONGPMkY6pQBJZXwBkpVhM4BA7buFQ1 1YggYDBLYWVxjZD3UjQJebP2/j/rsEw= 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-95-WIFaDxcFNmaQ_WrGwIpwSw-1; Fri, 19 Jun 2020 05:43:01 -0400 X-MC-Unique: WIFaDxcFNmaQ_WrGwIpwSw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 655D684B842 for ; Fri, 19 Jun 2020 09:43:00 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5DA9D5C1D6; Fri, 19 Jun 2020 09:42:51 +0000 (UTC) Date: Fri, 19 Jun 2020 05:42:50 -0400 From: Mike Snitzer To: Ming Lei 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: <20200619094250.GA18410@redhat.com> References: <20200619084214.337449-1-ming.lei@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200619084214.337449-1-ming.lei@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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? > 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. > 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? 2) the race you detail (with competing blk_mq_quiesce_queue) relative to synchronize_rcu() and testing "the flag" is very detailed yet vague. 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. 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 >