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 CDAC7C433FE for ; Fri, 4 Dec 2020 11:48:11 +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 71F6522A84 for ; Fri, 4 Dec 2020 11:48:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 71F6522A84 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.44633.79992 (Exim 4.92) (envelope-from ) id 1kl9ZE-0006Fa-4X; Fri, 04 Dec 2020 11:48:04 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 44633.79992; Fri, 04 Dec 2020 11:48:04 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kl9ZE-0006FS-1D; Fri, 04 Dec 2020 11:48:04 +0000 Received: by outflank-mailman (input) for mailman id 44633; Fri, 04 Dec 2020 11:48:02 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kl9ZB-0006F4-Ud for xen-devel@lists.xenproject.org; Fri, 04 Dec 2020 11:48:01 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id c2b01667-3aed-46b2-8eaa-97d939b7021a; Fri, 04 Dec 2020 11:48:00 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id F2AA0AC9A; Fri, 4 Dec 2020 11:47:59 +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: c2b01667-3aed-46b2-8eaa-97d939b7021a 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=1607082480; 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=kuaCdU3/SANic9kCA6C0IYdinVYmrdvcgidim9WiC+k=; b=ftHmDNIh0IEJwtUqUBdkGBYqBo/cuGVp1u9MNJqkHPOEUW8tHvcfxGgZc/TTGn4iiTLKnR ocoCjEUwupFHsCj12focqfbjBPf40dmn8Ecmk4k2pEbvxyT32VXDmqh91RnoUgP0Ig6D3R Frzh8SwJElq4jaN1oxBUpp+ILfCONMM= 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" 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> From: Jan Beulich Message-ID: <269f9a2d-7a8d-cba2-801f-6d3b12f9455f@suse.com> Date: Fri, 4 Dec 2020 12:48:00 +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: <1a8250f5-ea49-ac3a-e992-be7ec40deba9@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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? Or else what exactly am I to take from your reply? Jan