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 2E6CCC433FE for ; Mon, 7 Dec 2020 15:28:20 +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 D5DDA2371F for ; Mon, 7 Dec 2020 15:28:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D5DDA2371F Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com 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.46715.82817 (Exim 4.92) (envelope-from ) id 1kmIQs-0002Rq-BB; Mon, 07 Dec 2020 15:28:10 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 46715.82817; Mon, 07 Dec 2020 15:28:10 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmIQs-0002Re-87; Mon, 07 Dec 2020 15:28:10 +0000 Received: by outflank-mailman (input) for mailman id 46715; Mon, 07 Dec 2020 15:28:08 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kmIQq-0002QU-Fn for xen-devel@lists.xenproject.org; Mon, 07 Dec 2020 15:28:08 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 1e031b65-33ee-4af9-a51f-591451357b0d; Mon, 07 Dec 2020 15:28:07 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B6861ABE9; Mon, 7 Dec 2020 15:28:06 +0000 (UTC) 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" X-Inumbo-ID: 1e031b65-33ee-4af9-a51f-591451357b0d X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1607354886; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=JjN23h87VAmxj756ADXTtOqjPfVg45T7K9PwtwT1L+o=; b=WGYAfZnaY0j9dFXHlj3mNNmo4uGG5MytVOBM7BtHleS13j+HQOsNr7KH7cO18LXveGoKlY 9aMTaK3EQK7gc+7qPbqnrHodlas8Bi7B6Vfwm7VATrZGiqrJl3dvyp9wxxsIg6PxSoev/9 PfE6Q9ih+/IvCjLHkhGP2i/laqny9+Y= Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Julien Grall 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: Jan Beulich Message-ID: Date: Mon, 7 Dec 2020 16:28:08 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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? Jan