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=-2.2 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 4D947CA9EA0 for ; Fri, 25 Oct 2019 14:18:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1BA6120684 for ; Fri, 25 Oct 2019 14:18:10 +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="FteJT4cq" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729370AbfJYOSJ (ORCPT ); Fri, 25 Oct 2019 10:18:09 -0400 Received: from mail-il1-f194.google.com ([209.85.166.194]:42469 "EHLO mail-il1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726936AbfJYOSJ (ORCPT ); Fri, 25 Oct 2019 10:18:09 -0400 Received: by mail-il1-f194.google.com with SMTP id o16so1968730ilq.9 for ; Fri, 25 Oct 2019 07:18:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:from:to:cc:references:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=HwWQrehnQ6PqS1LXNfK7buSft8cUBtQIORQS/x4zW6I=; b=FteJT4cqk55kEwPSubwWqeYHDwFrHlql9PFbTO/R+ySJ1BaB/ZaSVT9Y9QgOVAPdRa Jw3mFBrHx39leSkOw+XFl1f0bEHBUme9REQto/rr/Z+lH5xlVDnaT1QCmun622RzlVnF +hrnkoYsY6XBRtcPuXumrGA4JDJMA794FGnoBe8AxbRNXz2PudiGu0lc4ltK1n+P7IdW xt/cyfmLcWh3CfKp9Lo90lqvU5JmVKuPHTC4SMsl6UWpFFQUIDezWYPPTxgikpJhllCf jO2zo1TfaaYQrrff9M04olq19ajq01llgL0W94InmoAveUZr/uiupqrx02oYUqzFIpZD iBjA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:from:to:cc:references:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=HwWQrehnQ6PqS1LXNfK7buSft8cUBtQIORQS/x4zW6I=; b=tcqhYIhbYC6hjanbMyLMHfMYz/fecn81EftXFe/ctW+2Mhcx/zPz/qqLHyPhRBJxHr RpUe5ZPVh103VpEpkt0KNZrMQAe0JEnfBDn+g960drVDnqsmojvlxPUQ8+kH5/TllV80 zYFEtgQrL3x3+tMVVNLXw1fmMV7DVyxOyeZZIFJl0PO0jC6Nrwc2wrbaGqFzjIyVZbJX N5zM7p2E83z0KtLCYoOpOwbKaMtsYYgH/3IQVu6Wl538IKblo7Zdhqg20DVuPbSsQ1EE Y+aZ4irKnOvjDxOUiLsmZhyMljLimmwcfUS6N0DsMgWZKBi1+24kBElb0Ys0TWVBCRrg 75hw== X-Gm-Message-State: APjAAAUbZRqoO+O6m+yEPEFbZKpKnlmYnNXMz464Jb1zMNO/lWQEx7PC mHOSkYFjPQ8ZfwV7h0PEpwSBa5+jH3oHhw== X-Google-Smtp-Source: APXvYqy6vL/Lf7GXQ7zcQqKQsFqwqXtUiYLW9EM6eiRT8kLIqDpzdr4xsKe6XOSnqm2+gxqF8pj+0A== X-Received: by 2002:a92:8746:: with SMTP id d6mr4440653ilm.265.1572013084035; Fri, 25 Oct 2019 07:18:04 -0700 (PDT) Received: from [192.168.1.159] ([65.144.74.34]) by smtp.gmail.com with ESMTPSA id c8sm275104iol.57.2019.10.25.07.18.02 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 25 Oct 2019 07:18:02 -0700 (PDT) Subject: Re: [RFC 0/2] io_uring: examine request result only after completion From: Jens Axboe 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> Message-ID: <533409a8-6907-44d8-1b90-a10ec3483c2c@kernel.dk> Date: Fri, 25 Oct 2019 08:18:01 -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 10/25/19 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. -- Jens Axboe