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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5C388C41513 for ; Tue, 15 Aug 2023 08:32:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235645AbjHOIca (ORCPT ); Tue, 15 Aug 2023 04:32:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53746 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235649AbjHOIcH (ORCPT ); Tue, 15 Aug 2023 04:32:07 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 00D6C127 for ; Tue, 15 Aug 2023 01:31:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1692088279; 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=znLlsPHHnv8BvLsndUsdokUxXtKvPNSjISxIiI4+uhc=; b=YIH7Wv4xcjQ3cq98dAyEj/IXzif+Ml3Il8IrkE0UGH6Zt25zBK7rTDnkw76qINLoLaEYwc tQD9zuP8jdlz5RILjrWctye8s8prp1TcEf9vXPhENj7QY9my6mrAkPbZFPiFWV1Dn4egbD G7M8NqdaIMIuWTXUxTIgc/f34KX3eB8= Received: from mimecast-mx02.redhat.com (66.187.233.73 [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-605-zXwMgMGBNT6zQ53C5Ok0sg-1; Tue, 15 Aug 2023 04:31:16 -0400 X-MC-Unique: zXwMgMGBNT6zQ53C5Ok0sg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7480D29A9CA9; Tue, 15 Aug 2023 08:31:15 +0000 (UTC) Received: from fedora (unknown [10.72.120.6]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5F795140E950; Tue, 15 Aug 2023 08:31:07 +0000 (UTC) Date: Tue, 15 Aug 2023 16:31:02 +0800 From: Ming Lei To: chengming.zhou@linux.dev Cc: axboe@kernel.dk, kch@nvidia.com, dhowells@redhat.com, damien.lemoal@opensource.wdc.com, bvanassche@acm.org, nj.shetty@samsung.com, kbusch@kernel.org, zhouchengming@bytedance.com, akinobu.mita@gmail.com, shinichiro.kawasaki@wdc.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] null_blk: fix poll request timeout handling Message-ID: References: <20230815060443.660263-1-chengming.zhou@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230815060443.660263-1-chengming.zhou@linux.dev> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 15, 2023 at 02:04:42PM +0800, chengming.zhou@linux.dev wrote: > From: Chengming Zhou > > When doing io_uring benchmark on /dev/nullb0, it's easy to crash the > kernel if poll requests timeout triggered, as reported by David. [1] Just be curious, how is the timeout triggered when running "./fio/t/io_uring -r20 /dev/nullb0"? David mentioned that the issue is triggered in 6.5-rc1, maybe one regression? > > BUG: kernel NULL pointer dereference, address: 0000000000000008 > Workqueue: kblockd blk_mq_timeout_work > RIP: 0010:null_timeout_rq+0x4e/0x91 > Call Trace: > ? __die_body+0x1a/0x5c > ? page_fault_oops+0x6f/0x9c > ? kernelmode_fixup_or_oops+0xc6/0xd6 > ? __bad_area_nosemaphore+0x44/0x1eb > ? exc_page_fault+0xe2/0xf4 > ? asm_exc_page_fault+0x22/0x30 > ? null_timeout_rq+0x4e/0x91 > blk_mq_handle_expired+0x31/0x4b > bt_iter+0x68/0x84 > ? bt_tags_iter+0x81/0x81 > __sbitmap_for_each_set.constprop.0+0xb0/0xf2 > ? __blk_mq_complete_request_remote+0xf/0xf > bt_for_each+0x46/0x64 > ? __blk_mq_complete_request_remote+0xf/0xf > ? percpu_ref_get_many+0xc/0x2a > blk_mq_queue_tag_busy_iter+0x14d/0x18e > blk_mq_timeout_work+0x95/0x127 > process_one_work+0x185/0x263 > worker_thread+0x1b5/0x227 > ? rescuer_thread+0x287/0x287 > kthread+0xfa/0x102 > ? kthread_complete_and_exit+0x1b/0x1b > ret_from_fork+0x22/0x30 > > This is indeed a race problem between null_timeout_rq() and null_poll(). > > null_poll() null_timeout_rq() > spin_lock(&nq->poll_lock) > list_splice_init(&nq->poll_list, &list) > spin_unlock(&nq->poll_lock) > > while (!list_empty(&list)) > req = list_first_entry() > list_del_init() > ... > blk_mq_add_to_batch() > // req->rq_next = NULL > spin_lock(&nq->poll_lock) > > // rq->queuelist->next == NULL > list_del_init(&rq->queuelist) > > spin_unlock(&nq->poll_lock) > > What's worse is that we don't call blk_mq_complete_request_remote() > before blk_mq_add_to_batch(), so these completed requests have wrong > rq->state == MQ_RQ_IN_FLIGHT. We can easily check this using bpftrace: > > ``` > bpftrace -e 'kretfunc:null_blk:null_poll { > $iob=(struct io_comp_batch *)args->iob; > @[$iob->req_list->state]=count(); > }' > > @[1]: 51708 > ``` > > Fix these problems by setting requests state to MQ_RQ_COMPLETE under > nq->poll_lock protection, in which null_timeout_rq() can safely detect > this race and early return. > > [1] https://lore.kernel.org/all/3893581.1691785261@warthog.procyon.org.uk/ > > Fixes: 0a593fbbc245 ("null_blk: poll queue support") > Reported-by: David Howells > Signed-off-by: Chengming Zhou > --- > drivers/block/null_blk/main.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c > index 864013019d6b..968090935eb2 100644 > --- a/drivers/block/null_blk/main.c > +++ b/drivers/block/null_blk/main.c > @@ -1643,9 +1643,12 @@ static int null_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob) > struct nullb_queue *nq = hctx->driver_data; > LIST_HEAD(list); > int nr = 0; > + struct request *rq; > > spin_lock(&nq->poll_lock); > list_splice_init(&nq->poll_list, &list); > + list_for_each_entry(rq, &list, queuelist) > + blk_mq_set_request_complete(rq); > spin_unlock(&nq->poll_lock); > > while (!list_empty(&list)) { > @@ -1671,16 +1674,21 @@ static enum blk_eh_timer_return null_timeout_rq(struct request *rq) > struct blk_mq_hw_ctx *hctx = rq->mq_hctx; > struct nullb_cmd *cmd = blk_mq_rq_to_pdu(rq); > > - pr_info("rq %p timed out\n", rq); > - > if (hctx->type == HCTX_TYPE_POLL) { > struct nullb_queue *nq = hctx->driver_data; > > spin_lock(&nq->poll_lock); > + /* The request may have completed meanwhile. */ > + if (blk_mq_request_completed(rq)) { > + spin_unlock(&nq->poll_lock); > + return BLK_EH_DONE; > + } > list_del_init(&rq->queuelist); > spin_unlock(&nq->poll_lock); > } I think null_process_cmd() is needed for un-completed request. Thanks, Ming