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 5E83BC4338F for ; Fri, 20 Aug 2021 22:09:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2726B61167 for ; Fri, 20 Aug 2021 22:09:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233044AbhHTWK1 (ORCPT ); Fri, 20 Aug 2021 18:10:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56810 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232982AbhHTWKZ (ORCPT ); Fri, 20 Aug 2021 18:10:25 -0400 Received: from mail-il1-x132.google.com (mail-il1-x132.google.com [IPv6:2607:f8b0:4864:20::132]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 63DF9C061575 for ; Fri, 20 Aug 2021 15:09:47 -0700 (PDT) Received: by mail-il1-x132.google.com with SMTP id s16so10908564ilo.9 for ; Fri, 20 Aug 2021 15:09:47 -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=3sqjf9VxLjK+MDaP0BXLm9lMJdVOX/YWtEut6X9GlT8=; b=c7GZ8O23erIiQgtV/h5773HKLiChI/JBQeRaJPQhXpQ/xr78Kk7erclCb2KehivRsm y4dVgbFYjItkKEgjSEebSRQAYrC4HSUQmjheEVV49jpp0rco1aqXOlFP4ODSYFtOyIIw 4J8hsDjxq80Dsst/Ad8Lb1OfCb4dd5JHv/bx1AZGEx70pastPv261f6IL9w2R0Zh7Wl8 ZG3lLrQmtDDQlgAyanORZ/x9h9HXOX2+9Lx3lUJlDMIBdATjSJ7kdLzWz86h5388Vkhy XuqmOAIK2poU4D2qcxUUu/3O6GeYmlz7qb5w7yUa489Jn27X/MRGBrkjOataoHTh1TbH oi2w== 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=3sqjf9VxLjK+MDaP0BXLm9lMJdVOX/YWtEut6X9GlT8=; b=WYYbFmjVU6d2rXHR+KdNVUaByqTaEMx1EdMWxCrVQ4IOThR3MDQpEk5CqAtdOwVAEp qMWsQMpHCdRCPTZ5wrQkFAhsrXR7lfFcpyx/sqc/LVj+gYuf9XRrJUcBclh5mtKms/dh TNI+FO6VHIlWVZoNFvbiWQjJuRDKG3dGvKt/q94BSHQ7KMsX3MT3JU08SUP8HQvgxJ8H VaI9kWzNQf1MQ1Ax07IGCH3HVcsC+Sy8K5MBq1X7L+3vuf4PxxvoSGCYnc8BlUCN3p4e UeSVdNFMzrZvcFNv7ZvWvXBPx74BSpQdtJ+7CyaPScN8P0r+IK+6nRuo9nJWQwK52+ZO h3Zw== X-Gm-Message-State: AOAM530rwHFk4DeXDNUyo3BR9/dMY6oWAvmAeB5rS8X89aY9LffDtf9s dB2YgxfWYjq3Q7VFOiae5g1YF9QhBK6tWnKy X-Google-Smtp-Source: ABdhPJyygP7t01JKfYBbC4yQb9sfnu8l8bix04DnE0BFSN8HlO+d6jTVnDnM4vv4+LHwsJ4SKo4/pg== X-Received: by 2002:a05:6e02:f91:: with SMTP id v17mr10112122ilo.0.1629497386803; Fri, 20 Aug 2021 15:09:46 -0700 (PDT) Received: from [192.168.1.116] ([66.219.217.159]) by smtp.gmail.com with ESMTPSA id s6sm3960905iow.1.2021.08.20.15.09.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Aug 2021 15:09:46 -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> From: Jens Axboe Message-ID: <77a44fce-c831-16a6-8e80-9aee77f496a2@kernel.dk> Date: Fri, 20 Aug 2021 16:09:44 -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: <68755d6f-8049-463f-f372-0ddc978a963e@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 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: 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 :-) Hao, care to spin a patch that does that? -- Jens Axboe