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=-6.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 BE6EFC4338F for ; Fri, 20 Aug 2021 22:30:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A29C6610E6 for ; Fri, 20 Aug 2021 22:30:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231482AbhHTWbM (ORCPT ); Fri, 20 Aug 2021 18:31:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33256 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231274AbhHTWbL (ORCPT ); Fri, 20 Aug 2021 18:31:11 -0400 Received: from mail-il1-x12a.google.com (mail-il1-x12a.google.com [IPv6:2607:f8b0:4864:20::12a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8701CC061575 for ; Fri, 20 Aug 2021 15:30:33 -0700 (PDT) Received: by mail-il1-x12a.google.com with SMTP id j15so10990298ila.1 for ; Fri, 20 Aug 2021 15:30:33 -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=gOaZyDFVIT12Yw4Y7QOv2itnmEmI5B+3V3kDWmYbtmk=; b=B6xEvSmGqyrsUJJ8iknSR3vzs8tVSsF4wptxpAAR9pPPGXwZxm28u9towTiyxfyraI ofVJkSPkZNwilWWWzefSEQvW5k1mwg6bCcjydw+fXE2Fy6awInqmXfiE0yeWlPNXmodg DjDVp4/OfKpODRPnx7RuSZiebmrAf7sNkG8GWiBHllmmBQgFLvNJEYc00O+7qnm4zKbq ONYnTuP21pEL+OzRZbVe/eN//0LLC2zBvEwbFLspf8EmMPuvSs03LiwgQO/4V1QN1XLM Y4U0soZzTzKOP6UjZJnPj5xzcPb6q6NPzB0QKq8cA1HUtqI7eR+GfalAiu66pER0s+fP Q/qQ== 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=gOaZyDFVIT12Yw4Y7QOv2itnmEmI5B+3V3kDWmYbtmk=; b=hw2DTsynqEFJkngRRFvV0/exACfRHQERxTfy/eaYxg63y4A1pNuOwT9LVrzAT5MG8u lZYOf9qELwtzj8bWTy/aL4Doput6F2XWx8hUixzJvUvXKirBorH3p5FZr67sg/ebL+Sv +JHAGQzVOXFAXj4xMtXukmUDWEPXCcwTWIGaIT455Gc/g8+vg8z64A6lahjd4k7ne0G8 6kxOHUR3+vFLYibdJpm7pOSPz3B+udzmCT1CpAWT51LvX5L1hRoR4W+DjZK7fhJBSFDU AnJZFgWnynZmhx0zrcrZ/kWAXwElBmxAECln4DQ5OtBe3pPnxYGkZ/X94H/oi8LhDGhQ kEEQ== X-Gm-Message-State: AOAM532VhAOgWG2NggNzzJPATQh39+GmLPdw05tWGeD9XSqy/bRopSOh y8g4E9Yh4lp4A7VF/pLrO0jDpw== X-Google-Smtp-Source: ABdhPJzJ/IsZxJ/ZYWWsVIc0N7wg8Kqzmj2cXbETvu6bLluZCWC5lH81akWtkNsE8BUNhbNVF3xGBg== X-Received: by 2002:a05:6e02:1b05:: with SMTP id i5mr920342ilv.5.1629498632896; Fri, 20 Aug 2021 15:30:32 -0700 (PDT) Received: from [192.168.1.116] ([66.219.217.159]) by smtp.gmail.com with ESMTPSA id z15sm3987977ioh.28.2021.08.20.15.30.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Aug 2021 15:30:32 -0700 (PDT) Subject: Re: [PATCH for-5.15] io_uring: fix lacking of protection for compl_nr To: Pavel Begunkov , Hao Xu Cc: io-uring@vger.kernel.org, Joseph Qi References: <20210820184013.195812-1-haoxu@linux.alibaba.com> <4b6d903b-09ec-0fca-fa70-4c6c32a0f5cb@linux.alibaba.com> <68755d6f-8049-463f-f372-0ddc978a963e@gmail.com> <77a44fce-c831-16a6-8e80-9aee77f496a2@kernel.dk> <8ab470e7-83f1-a0ef-f43b-29af8f84d229@gmail.com> From: Jens Axboe Message-ID: <3cae21c2-5db7-add1-1587-c87e22e726dc@kernel.dk> Date: Fri, 20 Aug 2021 16:30:30 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <8ab470e7-83f1-a0ef-f43b-29af8f84d229@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 8/20/21 4:28 PM, Pavel Begunkov wrote: > On 8/20/21 11:09 PM, Jens Axboe wrote: >> On 8/20/21 3:32 PM, Pavel Begunkov wrote: >>> On 8/20/21 9:39 PM, Hao Xu wrote: >>>> 在 2021/8/21 上午2:59, Pavel Begunkov 写道: >>>>> On 8/20/21 7:40 PM, Hao Xu wrote: >>>>>> coml_nr in ctx_flush_and_put() is not protected by uring_lock, this >>>>>> may cause problems when accessing it parallelly. >>>>> >>>>> Did you hit any problem? It sounds like it should be fine as is: >>>>> >>>>> The trick is that it's only responsible to flush requests added >>>>> during execution of current call to tctx_task_work(), and those >>>>> naturally synchronised with the current task. All other potentially >>>>> enqueued requests will be of someone else's responsibility. >>>>> >>>>> So, if nobody flushed requests, we're finely in-sync. If we see >>>>> 0 there, but actually enqueued a request, it means someone >>>>> actually flushed it after the request had been added. >>>>> >>>>> Probably, needs a more formal explanation with happens-before >>>>> and so. >>>> I should put more detail in the commit message, the thing is: >>>> say coml_nr > 0 >>>> >>>> ctx_flush_and put other context >>>> if (compl_nr) get mutex >>>> coml_nr > 0 >>>> do flush >>>> coml_nr = 0 >>>> release mutex >>>> get mutex >>>> do flush (*) >>>> release mutex >>>> >>>> in (*) place, we do a bunch of unnecessary works, moreover, we >>> >>> I wouldn't care about overhead, that shouldn't be much >>> >>>> call io_cqring_ev_posted() which I think we shouldn't. >>> >>> IMHO, users should expect spurious io_cqring_ev_posted(), >>> though there were some eventfd users complaining before, so >>> for them we can do >> >> It does sometimes cause issues, see: > > I'm used that locking may end up in spurious wakeups. May be > different for eventfd, but considering that we do batch > completions and so might be calling it only once per multiple > CQEs, it shouldn't be. The wakeups are fine, it's the ev increment that's causing some issues. >> commit b18032bb0a883cd7edd22a7fe6c57e1059b81ed0 >> Author: Jens Axboe >> Date: Sun Jan 24 16:58:56 2021 -0700 >> >> io_uring: only call io_cqring_ev_posted() if events were posted >> >> I would tend to agree with Hao here, and the usual optimization idiom >> looks like: >> >> if (struct->nr) { >> mutex_lock(&struct->lock); >> if (struct->nr) >> do_something(); >> mutex_unlock(&struct->lock); >> } >> >> like you posted, which would be fine and avoid this whole discussion :-) > > Well, until the Hao's message explaining the concerns, I was thinking > it's about potential hangs because of not flushing requests. I'd rather > say the discussion was fruitful and naturally came to the conclusion. Oh for sure, didn't mean to imply it was useless. At least it's in the archives :) -- Jens Axboe