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=-8.3 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,URIBL_BLOCKED, 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 25D46C433DF for ; Fri, 19 Jun 2020 17:40:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EFAAA207FC for ; Fri, 19 Jun 2020 17:40:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="OoATyPGL" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731428AbgFSRky (ORCPT ); Fri, 19 Jun 2020 13:40:54 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:54744 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730934AbgFSRkx (ORCPT ); Fri, 19 Jun 2020 13:40:53 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592588451; 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=SSpbTMXzbufsXH9rLr9CGShczMzOHa1HOXBaPDsWskY=; b=OoATyPGLiTw5xA/C+IVa9zr9t6CyzcINFbcJuBETxP6tMkXynzU5H8L9ABvR9+XeLaSXKq kdvy7pAbb2i40KKoelDlIbPdUuTjEKi1Sj9okBoAp9wFB1/hurqLaONCaoUdcBo5uN3CsT 3JhC2JyqdxrB9MprWMp+fVXW5pom2yk= 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-398-vwSGpqWLNw6k3V1hYXsTWA-1; Fri, 19 Jun 2020 13:40:49 -0400 X-MC-Unique: vwSGpqWLNw6k3V1hYXsTWA-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 43BF9872FED; Fri, 19 Jun 2020 17:40:48 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 308EB19D7B; Fri, 19 Jun 2020 17:40:42 +0000 (UTC) Date: Fri, 19 Jun 2020 13:40:41 -0400 From: Mike Snitzer To: Ming Lei Cc: dm-devel@redhat.com, linux-block@vger.kernel.org, axboe@kernel.dk, Bart Van Assche Subject: Re: dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue() Message-ID: <20200619174040.GA24968@redhat.com> References: <20200619084214.337449-1-ming.lei@redhat.com> <20200619094250.GA18410@redhat.com> <20200619101142.GA339442@T590> <20200619160657.GA24520@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200619160657.GA24520@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Jun 19 2020 at 12:06pm -0400, Mike Snitzer wrote: > On Fri, Jun 19 2020 at 6:11am -0400, > Ming Lei wrote: > > > 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. > > I'm going to look at actually stopping the queue (using one of these > interfaces). I didn't realize I wasn't actually stopping the queue. > The intent was to do so. > > In speaking with Jens yesterday about freeze vs stop: it is clear that > dm-rq needs to still be able to allocate new requests, but _not_ call > the queue_rq to issue the requests, while "stopped" (due to dm-mpath > potentially deferring retries of failed requests because of path failure > while quiescing the queue during DM device suspend). But that freezing > the queue goes too far because it won't allow such request allocation. Seems I'm damned if I do (stop) or damned if I don't (new reports of requests completing after DM device suspend's blk_mq_quiesce_queue()+dm_wait_for_completion()). I'm left at something of a loss about what to do! Bart? Jens? Ming? Looking closer at the git history, commit 7b17c2f7292ba takes center stage: commit 7b17c2f7292ba1f3f98dae3f7077f9e569653276 Author: Bart Van Assche Date: Fri Oct 28 17:22:16 2016 -0700 dm: Fix a race condition related to stopping and starting queues Ensure that all ongoing dm_mq_queue_rq() and dm_mq_requeue_request() calls have stopped before setting the "queue stopped" flag. This allows to remove the "queue stopped" test from dm_mq_queue_rq() and dm_mq_requeue_request(). This patch fixes a race condition because dm_mq_queue_rq() is called without holding the queue lock and hence BLK_MQ_S_STOPPED can be set at any time while dm_mq_queue_rq() is in progress. This patch prevents that the following hang occurs sporadically when using dm-mq: INFO: task systemd-udevd:10111 blocked for more than 480 seconds. Call Trace: [] schedule+0x37/0x90 [] schedule_timeout+0x27f/0x470 [] io_schedule_timeout+0x9f/0x110 [] bit_wait_io+0x16/0x60 [] __wait_on_bit_lock+0x49/0xa0 [] __lock_page+0xb9/0xc0 [] truncate_inode_pages_range+0x3e0/0x760 [] truncate_inode_pages+0x10/0x20 [] kill_bdev+0x30/0x40 [] __blkdev_put+0x71/0x360 [] blkdev_put+0x49/0x170 [] blkdev_close+0x20/0x30 [] __fput+0xe8/0x1f0 [] ____fput+0x9/0x10 [] task_work_run+0x83/0xb0 [] do_exit+0x3ee/0xc40 [] do_group_exit+0x4b/0xc0 [] get_signal+0x2ca/0x940 [] do_signal+0x23/0x660 [] exit_to_usermode_loop+0x73/0xb0 [] syscall_return_slowpath+0xb0/0xc0 [] entry_SYSCALL_64_fastpath+0xa6/0xa8 Signed-off-by: Bart Van Assche Acked-by: Mike Snitzer Reviewed-by: Hannes Reinecke Reviewed-by: Johannes Thumshirn Reviewed-by: Christoph Hellwig Signed-off-by: Jens Axboe diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c index 09c958b6f038..8b92e066bb69 100644 --- a/drivers/md/dm-rq.c +++ b/drivers/md/dm-rq.c @@ -102,7 +102,7 @@ static void dm_mq_stop_queue(struct request_queue *q) if (blk_mq_queue_stopped(q)) return; - blk_mq_stop_hw_queues(q); + blk_mq_quiesce_queue(q); } void dm_stop_queue(struct request_queue *q) @@ -880,17 +880,6 @@ static int dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx, dm_put_live_table(md, srcu_idx); } - /* - * On suspend dm_stop_queue() handles stopping the blk-mq - * request_queue BUT: even though the hw_queues are marked - * BLK_MQ_S_STOPPED at that point there is still a race that - * is allowing block/blk-mq.c to call ->queue_rq against a - * hctx that it really shouldn't. The following check guards - * against this rarity (albeit _not_ race-free). - */ - if (unlikely(test_bit(BLK_MQ_S_STOPPED, &hctx->state))) - return BLK_MQ_RQ_QUEUE_BUSY; - if (ti->type->busy && ti->type->busy(ti)) return BLK_MQ_RQ_QUEUE_BUSY;