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=-13.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 8BC04C433DB for ; Mon, 21 Dec 2020 17:45:50 +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 2AD1822B45 for ; Mon, 21 Dec 2020 17:45:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2AD1822B45 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.57484.100590 (Exim 4.92) (envelope-from ) id 1krPFY-00063H-Eb; Mon, 21 Dec 2020 17:45:36 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 57484.100590; Mon, 21 Dec 2020 17:45:36 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1krPFY-00063A-BK; Mon, 21 Dec 2020 17:45:36 +0000 Received: by outflank-mailman (input) for mailman id 57484; Mon, 21 Dec 2020 17:45:35 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1krPFW-000635-SZ for xen-devel@lists.xenproject.org; Mon, 21 Dec 2020 17:45:34 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1krPFW-0002Ka-7m; Mon, 21 Dec 2020 17:45:34 +0000 Received: from 54-240-197-234.amazon.com ([54.240.197.234] 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 1krPFV-0007fZ-Vw; Mon, 21 Dec 2020 17:45:34 +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=BkrRymnUmYJro9bAFzAgr6HO7qKXsBObkY72rMY6ntA=; b=C2XRBtonYOkohEPEkjehArLc2W iCeHr1+zqxe1y/MmAe+xFY+DfaiVe9zZy6nGVkI6JPN1/woqgcMmyeindLevJLBO6Hk6mSsBjVCzH 606mAStZiuJ74PytgTEpo44yebJHNkICg4r/HIA7h4eMK1SBv4q8VDqbnI0xjx4eNXao=; Subject: Re: [PATCH v3 4/5] evtchn: convert domain event lock to an r/w one To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , "xen-devel@lists.xenproject.org" References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <074be931-54b0-1b0f-72d8-5bd577884814@xen.org> <6e34fd25-14a2-f655-b019-aca94ce086c8@suse.com> <55dc24b4-88c6-1b22-411e-267231632377@xen.org> From: Julien Grall Message-ID: <1f3571eb-5aec-e76e-0b61-2602356fb436@xen.org> Date: Mon, 21 Dec 2020 17:45:32 +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 14/12/2020 09:40, Jan Beulich wrote: > On 11.12.2020 11:57, Julien Grall wrote: >> On 11/12/2020 10:32, Jan Beulich wrote: >>> On 09.12.2020 12:54, Julien Grall wrote: >>>> On 23/11/2020 13:29, Jan Beulich wrote: >>>>> @@ -620,7 +620,7 @@ int evtchn_close(struct domain *d1, int >>>>> long rc = 0; >>>>> >>>>> again: >>>>> - spin_lock(&d1->event_lock); >>>>> + write_lock(&d1->event_lock); >>>>> >>>>> if ( !port_is_valid(d1, port1) ) >>>>> { >>>>> @@ -690,13 +690,11 @@ int evtchn_close(struct domain *d1, int >>>>> BUG(); >>>>> >>>>> if ( d1 < d2 ) >>>>> - { >>>>> - spin_lock(&d2->event_lock); >>>>> - } >>>>> + read_lock(&d2->event_lock); >>>> >>>> This change made me realized that I don't quite understand how the >>>> rwlock is meant to work for event_lock. I was actually expecting this to >>>> be a write_lock() given there are state changed in the d2 events. >>> >>> Well, the protection needs to be against racing changes, i.e. >>> parallel invocations of this same function, or evtchn_close(). >>> It is debatable whether evtchn_status() and >>> domain_dump_evtchn_info() would better also be locked out >>> (other read_lock() uses aren't applicable to interdomain >>> channels). >>> >>>> Could you outline how a developper can find out whether he/she should >>>> use read_lock or write_lock? >>> >>> I could try to, but it would again be a port type dependent >>> model, just like for the per-channel locks. >> >> It is quite important to have clear locking strategy (in particular >> rwlock) so we can make correct decision when to use read_lock or write_lock. >> >>> So I'd like it to >>> be clarified first whether you aren't instead indirectly >>> asking for these to become write_lock() >> >> Well, I don't understand why this is a read_lock() (even with your >> previous explanation). I am not suggesting to switch to a write_lock(), >> but instead asking for the reasoning behind the decision. > > So if what I've said in my previous reply isn't enough (including the > argument towards using two write_lock() here), I'm struggling to > figure what else to say. The primary goal is to exclude changes to > the same ports. For this it is sufficient to hold just one of the two > locks in writer mode, as the other (racing) one will acquire that > same lock for at least reading. The question whether both need to use > writer mode can only be decided when looking at the sites acquiring > just one of the locks in reader mode (hence the reference to > evtchn_status() and domain_dump_evtchn_info()) - if races with them > are deemed to be a problem, switching to both-writers will be needed. I had another look at the code based on your explanation. I don't think it is fine to allow evtchn_status() to be concurrently called with evtchn_close(). evtchn_close() contains the following code: chn2->state = ECS_UNBOUND; chn2->u.unbound.remote_domid = d1->domain_id; Where chn2 is a event channel of the remote domain (d2). Your patch will only held the read lock for d2. However evtchn_status() expects the event channel state to not change behind its back. This assumption doesn't hold for d2, and you could possibly end up to see the new value of chn2->state after the new chn2->u.unbound.remote_domid. Thanksfully, it doesn't look like chn2->u.interdomain.remote_domain would be overwritten. Otherwise, this would be a straight dereference of an invalid pointer. So I think, we need to held the write event lock for both domain. Cheers, -- Julien Grall