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=-9.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 2A45CC433DB for ; Wed, 23 Dec 2020 15:08:35 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D0022207A4 for ; Wed, 23 Dec 2020 15:08:34 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D0022207A4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=xen.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.58428.102728 (Exim 4.92) (envelope-from ) id 1ks5kS-0007SN-3b; Wed, 23 Dec 2020 15:08:20 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 58428.102728; Wed, 23 Dec 2020 15:08:20 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1ks5kR-0007SG-Vs; Wed, 23 Dec 2020 15:08:19 +0000 Received: by outflank-mailman (input) for mailman id 58428; Wed, 23 Dec 2020 15:08:18 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1ks5kQ-0007SB-JO for xen-devel@lists.xenproject.org; Wed, 23 Dec 2020 15:08:18 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1ks5kO-0000X3-JO; Wed, 23 Dec 2020 15:08:16 +0000 Received: from [54.239.6.186] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1ks5kO-0002fb-BS; Wed, 23 Dec 2020 15:08:16 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=iGTyooH/uXRVYSjXj+Kacw5JiyS5fUAIh1te09HPLUI=; b=jbG7U1c6r5Pu9CCdW1zwDKlZmt bD4tVJZCpXz4io5Ze4P3oPLOVZlG1VNFI5a+u9R5MKipX2bD95DPSV1+eErEuKiMUSPs/9f65+pHq Vs9WoOWMdnV2IFt+bJ7xOOmXB/xqg6aUKy017Y9Zvu7vH3OUDi/kbdTDell9owc1sujk=; Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , Tamas K Lengyel , Petre Ovidiu PIRCALABU , Alexandru Isaila , "xen-devel@lists.xenproject.org" , Tamas K Lengyel References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org> <7a768bcd-80c1-d193-8796-7fb6720fa22a@suse.com> <1a8250f5-ea49-ac3a-e992-be7ec40deba9@xen.org> <5862eb24-d894-455a-13ac-61af54f949e7@xen.org> <9ee6016a-d3b3-c847-4775-0e05c8578110@xen.org> <3b339f30-57db-caf6-fd7e-84199f98546f@suse.com> <9c214bc1-61db-5b33-f610-40c2a59edb75@xen.org> From: Julien Grall Message-ID: <9b50863c-4236-7202-6c03-555e58cbf9ec@xen.org> Date: Wed, 23 Dec 2020 15:08:13 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi Jan, On 23/12/2020 14:56, Jan Beulich wrote: > On 23.12.2020 15:44, Julien Grall wrote: >> On 23/12/2020 13:41, Jan Beulich wrote: >>> On 23.12.2020 14:33, Julien Grall wrote: >>>> On 23/12/2020 13:12, Jan Beulich wrote: >>>>> From the input by both of you I still can't >>>>> conclude whether this patch should remain as is in v4, or revert >>>>> back to its v2 version. Please can we get this settled so I can get >>>>> v4 out? >>>> >>>> I haven't had time to investigate the rest of the VM event code to find >>>> other cases where this may happen. I still think there is a bigger >>>> problem in the VM event code, but the maintainer disagrees here. >>>> >>>> At which point, I see limited reason to try to paper over in the common >>>> code. So I would rather ack/merge v2 rather than v3. >>> >>> Since I expect Tamas and/or the Bitdefender folks to be of the >>> opposite opinion, there's still no way out, at least if "rather >>> ack" implies a nak for v3. >> >> The only way out here is for someone to justify why this patch is >> sufficient for the VM event race. > > I think this is too much you demand. I guess you didn't notice that I did most of the job by providing an analysis in my e-mail... I don't think it is too much demanding to read the analysis and say whether I am correct or not. Do you really prefer to add code would get rot because unused? > Moving in the right direction > without arriving at the final destination is still a worthwhile > thing to do imo. > >>> Personally, if this expectation of >>> mine is correct, I'd prefer to keep the accounting but make it >>> optional (as suggested in a post-commit-message remark). >>> That'll eliminate the overhead you appear to be concerned of, >>> but of course it'll further complicate the logic (albeit just >>> slightly). >> >> I am more concerned about adding over complex code that would just >> papering over a bigger problem. I also can't see use of it outside of >> the VM event discussion. > > I think it is a generally appropriate thing to do to wait for > callbacks to complete before tearing down their origin control > structure. There may be cases where code structure makes this > unnecessary, but I don't think this can be an expectation to > all the users of the functionality. Hence my suggestion to > possibly make this optional (driven directly or indirectly by > the user of the registration function). As you tend to say, we should not add code unless there is a user. So far the only possible user is dubbious. If you find another user, then we can discuss whether this patch makes sense. Cheers, -- Julien Grall