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=-7.2 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 834F4C4361B for ; Mon, 7 Dec 2020 17:30:58 +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 2B8352388D for ; Mon, 7 Dec 2020 17:30:57 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2B8352388D 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.46881.83059 (Exim 4.92) (envelope-from ) id 1kmKLV-000103-Dj; Mon, 07 Dec 2020 17:30:45 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 46881.83059; Mon, 07 Dec 2020 17:30:45 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmKLV-0000zv-Ai; Mon, 07 Dec 2020 17:30:45 +0000 Received: by outflank-mailman (input) for mailman id 46881; Mon, 07 Dec 2020 17:30:43 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmKLT-0000zq-LQ for xen-devel@lists.xenproject.org; Mon, 07 Dec 2020 17:30:43 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmKLR-0006Uy-MA; Mon, 07 Dec 2020 17:30:41 +0000 Received: from [54.239.6.187] (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 1kmKLR-0004M7-Ap; Mon, 07 Dec 2020 17:30:41 +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=qYLK5Rbilo17L0PX4ORzWdQytoszAoKkLyoSTL8bCJg=; b=ZXiNXYpF2OJlYRyFi8Jrrwwh40 aj4taQxfI4S7mT9vFF7g2hdUyhtY876jKqgGJrZRaTsW0NDV/XdIoCwysw+we8c0IM/glJKpvPWoM 6Dkoa2Wt4L43zDr5rhKiz50Fp4Vp7DXYQYrwv3cqWfMrvGwQQb3qPSf95CothjzEqqJc=; 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> From: Julien Grall Message-ID: <9ee6016a-d3b3-c847-4775-0e05c8578110@xen.org> Date: Mon, 7 Dec 2020 17:30:38 +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: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi Jan, On 07/12/2020 15:28, Jan Beulich wrote: > On 04.12.2020 20:15, Tamas K Lengyel wrote: >> On Fri, Dec 4, 2020 at 10:29 AM Julien Grall wrote: >>> On 04/12/2020 15:21, Tamas K Lengyel wrote: >>>> On Fri, Dec 4, 2020 at 6:29 AM Julien Grall wrote: >>>>> 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. >>>> >>>> I double-checked and the disable route is actually more robust, we >>>> don't just rely on the toolstack doing the right thing. The domain >>>> gets paused before any calls to vm_event_disable. So I don't think >>>> there is really a race-condition here. >>> >>> The code will *only* pause the monitored domain. I can see two issues: >>> 1) The toolstack is still sending event while destroy is happening. >>> This is the race discussed here. >>> 2) The implement of vm_event_put_request() suggests that it can be >>> called with not-current domain. >>> >>> I don't see how just pausing the monitored domain is enough here. >> >> Requests only get generated by the monitored domain. So if the domain >> is not running you won't get more of them. The toolstack can only send >> replies. > > Julien, > > does this change your view on the refcounting added by the patch > at the root of this sub-thread? I still think the code is at best fragile. One example I can find is: -> guest_remove_page() -> p2m_mem_paging_drop_page() -> vm_event_put_request() guest_remove_page() is not always call on the current domain. So there are possibility for vm_event_put_request() to happen on a foreign domain and therefore wouldn't be protected by the current hypercall. Anyway, I don't think the refcounting should be part of the event channel without any idea on how this would fit in fixing the VM event race. Cheers, -- Julien Grall