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 3E7C0C47082 for ; Mon, 7 Jun 2021 18:15:16 +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 DC7666100B for ; Mon, 7 Jun 2021 18:15:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DC7666100B 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.138135.255792 (Exim 4.92) (envelope-from ) id 1lqJmD-00036c-3z; Mon, 07 Jun 2021 18:15:05 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 138135.255792; Mon, 07 Jun 2021 18:15:05 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lqJmD-00036V-0t; Mon, 07 Jun 2021 18:15:05 +0000 Received: by outflank-mailman (input) for mailman id 138135; Mon, 07 Jun 2021 18:15:04 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lqJmC-00036P-6N for xen-devel@lists.xenproject.org; Mon, 07 Jun 2021 18:15:04 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lqJmB-0006EC-A1; Mon, 07 Jun 2021 18:15:03 +0000 Received: from 54-240-197-227.amazon.com ([54.240.197.227] 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 1lqJmB-0002fS-2o; Mon, 07 Jun 2021 18:15:03 +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=4pZL2seXSV736s975wjnJUTLCYC0Y6k2UPK2Kwx12pM=; b=u0HI7TbNJZKrXFw/RhcZ29HBnu UKK6J943icGkgGWM6RZK4+nKtt7HXegDLJ8SMtjVcoGM8Exf66mlHLDFvwiMVbQZcWTwNHdaLQozM 2AT2oype6/D0ugE4ICe7N2VK0fAJQUq39ANnywDpQv2cGEcQazON9CCnlPBEq01r0Cxo=; Subject: Re: [PATCH v6 1/3] evtchn: slightly defer lock acquire where possible To: Jan Beulich , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , "xen-devel@lists.xenproject.org" References: <01bbf3d4-ca6a-e837-91fe-b34aa014564c@suse.com> <5939858e-1c7c-5658-bc2d-0c9024c74040@suse.com> <938eb888-ec15-feb1-19f7-b90dfee822ae@xen.org> From: Julien Grall Message-ID: Date: Mon, 7 Jun 2021 19:15:01 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.2 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 01/06/2021 12:54, Jan Beulich wrote: > On 27.05.2021 20:48, Julien Grall wrote: >> On 27/05/2021 12:28, Jan Beulich wrote: >>> port_is_valid() and evtchn_from_port() are fine to use without holding >>> any locks. Accordingly acquire the per-domain lock slightly later in >>> evtchn_close() and evtchn_bind_vcpu(). >> >> So I agree that port_is_valid() and evtchn_from_port() are fine to use >> without holding any locks in evtchn_bind_vcpu(). However, this is >> misleading to say there is no problem with evtchn_close(). >> >> evtchn_close() can be called with current != d and therefore, there is a >> risk that port_is_valid() may be valid and then invalid because >> d->valid_evtchns is decremented in evtchn_destroy(). > > While this is the case for other functions as well (and hence a > comment along the lines of what you ask for below should have > been in place already), I've added > > /* > * While calling the function is okay without holding a suitable lock yet > * (see the comment ahead of struct evtchn_port_ops for which ones those > * are), for a dying domain it may start returning false at any point - see > * evtchn_destroy(). This is not a fundamental problem though, as the > * struct evtchn instance won't disappear (and will continue to hold valid > * data) until final cleanup of the domain, at which point the domain itself > * cannot be looked up anymore and hence calls here can't occur anymore in > * the first place. > */ > > ... > >> Thankfully the memory is still there. So the current code is okayish and >> I could reluctantly accept this behavior to be spread. However, I don't >> think this should be left uncommented in both the code (maybe on top of >> port_is_valid()?) and the commit message. > > ... ahead of port_is_valid() (and not, as I did intend originally, > in evtchn_close()). As far as the commit message goes, I'll have it > refer to the comment only. > > I hope this satisfies the requests of both of you. I'll take the > liberty and retain your ack, Roger. Yes, this satistfies my requests. Feel free to add my reviewed-by on the patch. Cheers, -- Julien Grall