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=-7.3 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,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 6E137C433FE for ; Fri, 4 Dec 2020 15:09:24 +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 2762F22B2D for ; Fri, 4 Dec 2020 15:09:24 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2762F22B2D 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.44881.80300 (Exim 4.92) (envelope-from ) id 1klChp-0004G8-19; Fri, 04 Dec 2020 15:09:09 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 44881.80300; Fri, 04 Dec 2020 15:09:09 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1klCho-0004G1-Tr; Fri, 04 Dec 2020 15:09:08 +0000 Received: by outflank-mailman (input) for mailman id 44881; Fri, 04 Dec 2020 15:09:07 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1klChn-0004Fw-GD for xen-devel@lists.xenproject.org; Fri, 04 Dec 2020 15:09:07 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1klChl-0004x8-D9; Fri, 04 Dec 2020 15:09:05 +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 1klChl-0004uC-2P; Fri, 04 Dec 2020 15:09:05 +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=Qt946/pZlmGRGP+S0I4pKTAzUdL3v8DCmAnKyp0CyBI=; b=YprXC9FV0wJqOnjWnslpaK5bXl xonwaHZqJKfSTvzEqVKHS7c4Wep1JAxnPmzphiNtyETTal9CR6alwuME2+oWymYXIVn0Aqd4pYnHB QC99RGoTAbrqsDh0klsC73ZiTESNZEl61vHTAXemQA2MOEioGcyNe64HlgdyQO78IbAU=; 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" 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> <269f9a2d-7a8d-cba2-801f-6d3b12f9455f@suse.com> <02a2b77f-27a9-b1b6-1acf-1f136cffdf30@xen.org> <48395363-ea47-9139-011e-233d92581a71@suse.com> From: Julien Grall Message-ID: <2edfc711-d8d9-4854-94a2-2d9e4d9902ec@xen.org> Date: Fri, 4 Dec 2020 15:09:02 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <48395363-ea47-9139-011e-233d92581a71@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi, On 04/12/2020 12:01, Jan Beulich wrote: > On 04.12.2020 12:51, Julien Grall wrote: >> >> >> On 04/12/2020 11:48, Jan Beulich wrote: >>> On 04.12.2020 12:28, Julien Grall wrote: >>>> Hi Jan, >>>> >>>> On 03/12/2020 10:09, Jan Beulich wrote: >>>>> On 02.12.2020 22:10, Julien Grall wrote: >>>>>> On 23/11/2020 13:30, Jan Beulich wrote: >>>>>>> While there don't look to be any problems with this right now, the lock >>>>>>> order implications from holding the lock can be very difficult to follow >>>>>>> (and may be easy to violate unknowingly). The present callbacks don't >>>>>>> (and no such callback should) have any need for the lock to be held. >>>>>>> >>>>>>> However, vm_event_disable() frees the structures used by respective >>>>>>> callbacks and isn't otherwise synchronized with invocations of these >>>>>>> callbacks, so maintain a count of in-progress calls, for evtchn_close() >>>>>>> to wait to drop to zero before freeing the port (and dropping the lock). >>>>>> >>>>>> AFAICT, this callback is not the only place where the synchronization is >>>>>> missing in the VM event code. >>>>>> >>>>>> For instance, vm_event_put_request() can also race against >>>>>> vm_event_disable(). >>>>>> >>>>>> So shouldn't we handle this issue properly in VM event? >>>>> >>>>> I suppose that's a question to the VM event folks rather than me? >>>> >>>> Yes. From my understanding of Tamas's e-mail, they are relying on the >>>> monitoring software to do the right thing. >>>> >>>> I will refrain to comment on this approach. However, given the race is >>>> much wider than the event channel, I would recommend to not add more >>>> code in the event channel to deal with such problem. >>>> >>>> Instead, this should be fixed in the VM event code when someone has time >>>> to harden the subsystem. >>> >>> Are effectively saying I should now undo the addition of the >>> refcounting, which was added in response to feedback from you? >> >> Please point out where I made the request to use the refcounting... > > You didn't ask for this directly, sure, but ... > >> I pointed out there was an issue with the VM event code. > > ... this has ultimately led to the decision to use refcounting > (iirc there was one alternative that I had proposed, besides > the option of doing nothing). One other option that was discussed (maybe only on security@xen.org) is to move the spinlock outside of the structure so it is always allocated. > >> This was latter >> analysed as a wider issue. The VM event folks doesn't seem to be very >> concerned on the race, so I don't see the reason to try to fix it in the >> event channel code. > > And you won't need the refcount for vpl011 then? I don't believe we need it for the vpl011 as the spin lock protecting the code should always be allocated. The problem today is the lock is initialized too late. > I can certainly > drop it again, but it feels odd to go back to an earlier version > under the circumstances ... The code introduced doesn't look necessary outside of the VM event code. So I think it would be wrong to merge it if it is just papering over a bigger problem. Cheers, > > Jan > -- Julien Grall