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,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 13427C4361A for ; Fri, 4 Dec 2020 11:52:01 +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 BEBDD22482 for ; Fri, 4 Dec 2020 11:52:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BEBDD22482 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.44649.80006 (Exim 4.92) (envelope-from ) id 1kl9ck-0007NQ-Nc; Fri, 04 Dec 2020 11:51:42 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 44649.80006; Fri, 04 Dec 2020 11:51:42 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kl9ck-0007NJ-Kl; Fri, 04 Dec 2020 11:51:42 +0000 Received: by outflank-mailman (input) for mailman id 44649; Fri, 04 Dec 2020 11:51:42 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kl9ck-0007ND-3I for xen-devel@lists.xenproject.org; Fri, 04 Dec 2020 11:51:42 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kl9ci-0000Pl-BI; Fri, 04 Dec 2020 11:51:40 +0000 Received: from [54.239.6.188] (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 1kl9ch-0005D2-UC; Fri, 04 Dec 2020 11:51:40 +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=1er+zurBPy0ur3esjX4CCRQUfXDV4JwhhAL7uyRCjd8=; b=TmFOgOSJa+vVyCjtOSPeEIdZW3 zoeuzomOOZ6Aq4NniXE7Ri6UoMfIs00afZSjR0dJVhzz/8Kkv1mDSUaaIXBqKq98/3IN1nEJ27G6E 6j5tmkuFrDQjeYukmGnRz8oJ4zBepBdJ/WIox4zkPBSMdF1H5iY+ZJ/3UX9KzG4NA4Yo=; 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> From: Julien Grall Message-ID: <02a2b77f-27a9-b1b6-1acf-1f136cffdf30@xen.org> Date: Fri, 4 Dec 2020 11:51:37 +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: <269f9a2d-7a8d-cba2-801f-6d3b12f9455f@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit 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... I pointed out there was an issue with the VM event code. 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. Cheers, -- Julien Grall