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=-2.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, 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 44509CA9EAE for ; Tue, 29 Oct 2019 19:33:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 09FF020862 for ; Tue, 29 Oct 2019 19:33:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=kernel-dk.20150623.gappssmtp.com header.i=@kernel-dk.20150623.gappssmtp.com header.b="iQbvvfIJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730404AbfJ2TdZ (ORCPT ); Tue, 29 Oct 2019 15:33:25 -0400 Received: from mail-io1-f49.google.com ([209.85.166.49]:32997 "EHLO mail-io1-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729253AbfJ2TdZ (ORCPT ); Tue, 29 Oct 2019 15:33:25 -0400 Received: by mail-io1-f49.google.com with SMTP id n17so3015479ioa.0 for ; Tue, 29 Oct 2019 12:33:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=StbTVtf5cfUUWJiwsyNhiwef4iErPHwHZprLy3XFx3M=; b=iQbvvfIJxSCqtQsHQS0r2QyIby8hRoSpjn8ahSLrHlPL3yfpK/l3BDQaZ4Xc47MBP+ MEgPNtu/NfDbpJcveiAsgQ9+yPn5Bcsb22EJsxbPVcpx5Ol1GLnftQ/VQeFW4wF62N5u zoIF9qnQqsrfClq0vRvoVrygEcUmTyIwAMMfPtqwY4px4yU4xaAJe7uYtnbs4mEpMwhu XRQxGbbW8X5AdvYt23CK6sCJufZPOA+OkcGDrgyg5o/OCnXlgbYOgUkfsSoqMIk1BE54 xzMWYF5J5D1NxNYrNaL9+qmAlDN9r7hN/j9zqTvvxfJH8zGJgtvgTvNj2LJR/ormx08m 0bCQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=StbTVtf5cfUUWJiwsyNhiwef4iErPHwHZprLy3XFx3M=; b=QPBLGEm1sFBx9nV0ICpttHv08MQKbOJs2smdWEaHwx9juh4Ub6l7GhxXXv4aKMlSk4 dgrV+Q/GUGsQuApjPKxpi8tm0xgvcBUs9DxzVJl6gGEErctVrK/vzjA0s3m+0ltHn7Uu mscq5tJPMAlqY+FWUlFUacbOE2d3v70fhmuaMVDYqLS7NvrZiKCoKfvMvYMpIMZKCm57 nhyqsAkwUagzGT/AM5USkaacITUQ8Cn3pKh8qarZVNr1Axmu0ZleVTs+aNIaVun4B6IE qXJCOTCMLX2vO2q2/loLeBNCvPuOsZzEORPjA3FlnJkccbnkXghovR57YRbEsVXAj179 awOw== X-Gm-Message-State: APjAAAUZSp2HntT/1qPQS2/GvxrNb7mE05GqLQjckKKTXu7T5ZTbeVFR 7abPALF6++XAVjY7D8hF4SDyZsaitTiZ5g== X-Google-Smtp-Source: APXvYqzqb3fkeXcj6Z4IbqRTTPhyqduccudxmclpWwUtFr1ZX/dXT1Og1Nme4WDHtIhbA8eaQiMdHg== X-Received: by 2002:a02:1d04:: with SMTP id 4mr1607294jaj.48.1572377603489; Tue, 29 Oct 2019 12:33:23 -0700 (PDT) Received: from [192.168.1.159] ([65.144.74.34]) by smtp.gmail.com with ESMTPSA id i79sm570037ild.6.2019.10.29.12.33.21 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 29 Oct 2019 12:33:22 -0700 (PDT) Subject: Re: [RFC 0/2] io_uring: examine request result only after completion To: Bijan Mottahedeh 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> <6adb9d2d-93f1-f915-7f20-5faa34b06398@kernel.dk> <34f483d9-2a97-30c3-9937-d3596649356c@oracle.com> <47b38d9d-04a3-99f6-c586-e82611d21655@kernel.dk> From: Jens Axboe Message-ID: <057bb6f9-29ec-1160-a1b1-00c57b610282@kernel.dk> Date: Tue, 29 Oct 2019 13:33:21 -0600 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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 10/29/19 1:31 PM, Bijan Mottahedeh wrote: > > On 10/29/19 12:27 PM, Jens Axboe wrote: >> On 10/29/19 1:23 PM, Bijan Mottahedeh wrote: >>> On 10/29/19 12:17 PM, Bijan Mottahedeh wrote: >>>> On 10/25/19 7:21 AM, Jens Axboe wrote: >>>>> On 10/25/19 8:18 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. >>>>> Ah, but we still have fops->iopoll() set for that case. So we just won't >>>>> poll for it, it'll get completed by IRQ. So I do think we need to handle >>>>> this case in io_uring. I'll get back to you. >>>>> >>>> I ran the same test on linux-next-20191029 in polled mode and got the >>>> same free-after-user panic: >>>> >>>> - I booted with nvme.poll_queues set and verified that all queues >>>> except default where of type poll >>>> >>>> - I added three assertions to verify the following: >>>> >>>> - nvme_timeout() is not called >>>> >>>> - io_complete_rw_iopoll() is not called from interrupt context >>>> >>>> - io_sq_offload_start() is not called with IORING_SETUP_SQPOLL set >>>> >>>> Is it possible that the race is there also in polled mode since a >>>> request submitted by one thread could conceivably be polled for and >>>> completed by a different thread, e.g. in >>>> io_uring_enter()->io_iopoll_check()? >>>> >>>> --bijan >>>> >>>> >>> I also tested my RFC again with 1 thread and with queue depths of 1 to >>> 1024 in multiples of 8 and didn't see any hangs. >>> >>> Just to be clear, the busy-spin logic discussed before was only a >>> workaround an not in the RFC. >> What is your exact test case? >> > See original cover letter. I can reproduce the failure with numjobs > between 8 and 32. And how many poll queues are you using? -- Jens Axboe