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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 8D5AFCA9EA0 for ; Fri, 25 Oct 2019 14:43:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5490C205ED for ; Fri, 25 Oct 2019 14:43:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="c21wMSUl" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2440485AbfJYOnE (ORCPT ); Fri, 25 Oct 2019 10:43:04 -0400 Received: from aserp2120.oracle.com ([141.146.126.78]:39640 "EHLO aserp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2440461AbfJYOnE (ORCPT ); Fri, 25 Oct 2019 10:43:04 -0400 Received: from pps.filterd (aserp2120.oracle.com [127.0.0.1]) by aserp2120.oracle.com (8.16.0.27/8.16.0.27) with SMTP id x9PEd2Vw024242; Fri, 25 Oct 2019 14:43:01 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2019-08-05; bh=5RRu3+gr6X25WegTfi/1/qk+bKkDm8bHsLu4oBL8FvM=; b=c21wMSUlOSEYVfjxqElj/rwdnvnYaj416tf9sFPOQiZAzQnNG90HsnfnzzkRp5AmSMum CDdtJK0Il1+zGse6A+CkUEyTMl2XEqZh1I/nOaEKe0byKxxtH4a2ZzMnbzKcYuMlMccU 73EegiuVwfcTMVv43PZ4t/rFKc+P+YffroN6BGR4Zcj+9pUNnQ4Hpdufpb5y3t4IrdhA lnV1FLPS5a0kuYjKthtaTW6Lqmh0NJKLuC6HKsWZewrBLIYOCinsAGB49WbJZbmX33Rz uNdHKPjHl0ozOdVUoI9e8cwXHyohUni6kXS0ikGt6afrMVUqzPXjbXO7r37RtWn2vW1Y iA== Received: from aserp3030.oracle.com (aserp3030.oracle.com [141.146.126.71]) by aserp2120.oracle.com with ESMTP id 2vqteqbhhh-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 25 Oct 2019 14:43:00 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.27/8.16.0.27) with SMTP id x9PEcwwY143905; Fri, 25 Oct 2019 14:43:00 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserp3030.oracle.com with ESMTP id 2vunbmj7ys-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 25 Oct 2019 14:43:00 +0000 Received: from abhmp0021.oracle.com (abhmp0021.oracle.com [141.146.116.27]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x9PEgwIs031006; Fri, 25 Oct 2019 14:42:59 GMT Received: from [10.175.3.200] (/10.175.3.200) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 25 Oct 2019 14:42:57 +0000 Subject: Re: [RFC 0/2] io_uring: examine request result only after completion To: Jens Axboe Cc: linux-block@vger.kernel.org References: <1571908688-22488-1-git-send-email-bijan.mottahedeh@oracle.com> <22fc1057-237b-a9b8-5a57-b7c53166a609@kernel.dk> <201931df-ae22-c2fc-a9c7-496ceb87dff7@oracle.com> <90c23805-4b73-4ade-1b54-dc68010b54dd@kernel.dk> <533409a8-6907-44d8-1b90-a10ec3483c2c@kernel.dk> From: Bijan Mottahedeh Message-ID: <37fee9db-484c-2ae2-08a1-c0681485ef57@oracle.com> Date: Fri, 25 Oct 2019 07:42:55 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <533409a8-6907-44d8-1b90-a10ec3483c2c@kernel.dk> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9420 signatures=668684 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=2 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1908290000 definitions=main-1910250140 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9420 signatures=668684 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1908290000 definitions=main-1910250140 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 10/25/19 7:18 AM, Jens Axboe wrote: > On 10/25/19 8:07 AM, Jens Axboe wrote: >> On 10/25/19 7:46 AM, Bijan Mottahedeh wrote: >>> On 10/24/19 3:31 PM, Jens Axboe wrote: >>>> On 10/24/19 1:18 PM, Bijan Mottahedeh wrote: >>>>> On 10/24/19 10:09 AM, Jens Axboe wrote: >>>>>> On 10/24/19 3:18 AM, Bijan Mottahedeh wrote: >>>>>>> Running an fio test consistenly crashes the kernel with the trace included >>>>>>> below. The root cause seems to be the code in __io_submit_sqe() that >>>>>>> checks the result of a request for -EAGAIN in polled mode, without >>>>>>> ensuring first that the request has completed: >>>>>>> >>>>>>> if (ctx->flags & IORING_SETUP_IOPOLL) { >>>>>>> if (req->result == -EAGAIN) >>>>>>> return -EAGAIN; >>>>>> I'm a little confused, because we should be holding the submission >>>>>> reference to the request still at this point. So how is it going away? >>>>>> I must be missing something... >>>>> I don't think the submission reference is going away... >>>>> >>>>> I *think* the problem has to do with the fact that >>>>> io_complete_rw_iopoll() which sets REQ_F_IOPOLL_COMPLETED is being >>>>> called from interrupt context in my configuration and so there is a >>>>> potential race between updating the request there and checking it in >>>>> __io_submit_sqe(). >>>>> >>>>> My first workaround was to simply poll for REQ_F_IOPOLL_COMPLETED in the >>>>> code snippet above: >>>>> >>>>> if (req->result == --EAGAIN) { >>>>> >>>>> poll for REQ_F_IOPOLL_COMPLETED >>>>> >>>>> return -EAGAIN; >>>>> >>>>> } >>>>> >>>>> and that got rid of the problem. >>>> But that will not work at all for a proper poll setup, where you don't >>>> trigger any IRQs... It only happens to work for this case because you're >>>> still triggering interrupts. But even in that case, it's not a real >>>> solution, but I don't think that's the argument here ;-) >>> Sure. >>> >>> I'm just curious though as how it would break the poll case because >>> io_complete_rw_iopoll() would still be called though through polling, >>> REQ_F_IOPOLL_COMPLETED would be set, and so io_iopoll_complete() >>> should be able to reliably check req->result. >> It'd break the poll case because the task doing the submission is >> generally also the one that finds and reaps completion. Hence if you >> block that task just polling on that completion bit, you are preventing >> that very task from going and reaping completions. The condition would >> never become true, and you are now looping forever. >> >>> The same poll test seemed to run ok with nvme interrupts not being >>> triggered. Anyway, no argument that it's not needed! >> A few reasons why it would make progress: >> >> - You eventually trigger a timeout on the nvme side, as blk-mq finds the >> request hasn't been completed by an IRQ. But that's a 30 second ordeal >> before that event occurs. >> >> - There was still interrupts enabled. >> >> - You have two threads, one doing submission and one doing completions. >> Maybe using SQPOLL? If that's the case, then yes, it'd still work as >> you have separate threads for submission and completion. >> >> For the "generic" case of just using one thread and IRQs disabled, it'd >> deadlock. >> >>>> I see what the race is now, it's specific to IRQ driven polling. We >>>> really should just disallow that, to be honest, it doesn't make any >>>> sense. But let me think about if we can do a reasonable solution to this >>>> that doesn't involve adding overhead for a proper setup. >>> It's a nonsensical config in a way and so disallowing it would make >>> the most sense. >> Definitely. The nvme driver should not set .poll() if it doesn't have >> non-irq poll queues. Something like this: > Actually, we already disable polling if we don't have specific poll > queues: > > if (set->nr_maps > HCTX_TYPE_POLL && > set->map[HCTX_TYPE_POLL].nr_queues) > blk_queue_flag_set(QUEUE_FLAG_POLL, q); > > Did you see any timeouts in your tests? I wonder if the use-after-free > triggered when the timeout found the request while you had the busy-spin > logic we discussed previously. > I didn't notice any timeouts.  The error triggered without me making any changes.  The busy-spin workaround I mentioned before actually got rid of the error. --bijan