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=-14.8 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 6D8BEC433DB for ; Tue, 22 Dec 2020 09:47:30 +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 27A402310D for ; Tue, 22 Dec 2020 09:47:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 27A402310D 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.57699.101028 (Exim 4.92) (envelope-from ) id 1kreFu-0005bF-Sr; Tue, 22 Dec 2020 09:46:58 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 57699.101028; Tue, 22 Dec 2020 09:46:58 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kreFu-0005b8-Pn; Tue, 22 Dec 2020 09:46:58 +0000 Received: by outflank-mailman (input) for mailman id 57699; Tue, 22 Dec 2020 09:46:57 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kreFt-0005b3-FL for xen-devel@lists.xenproject.org; Tue, 22 Dec 2020 09:46:57 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 20ed7f96-7dd8-48ea-95a9-9d40055911d3; Tue, 22 Dec 2020 09:46:56 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 33A1CAEA2; Tue, 22 Dec 2020 09:46:55 +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: 20ed7f96-7dd8-48ea-95a9-9d40055911d3 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=1608630415; 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=vH2tsfNS/wT6yUcJmU1L7Ai6jugWwSqwfdZVi1IqJhc=; b=gqKfdkhDaenTpkNZ8qTYQmLJfoKZ8VMqf+emmcAqbmxosZ7K170iMVto4oq76sRP0VMnWR 1uXrWFpPexnTbieMhnjc81Y5XvNq2l+ky3OQWQXKAEu822PuyFzGs788ifW98jLqUMG2cC Aq/vYMHAVG+Gh2ggQCOjm42ujgYNT/0= Subject: Re: [PATCH v3 4/5] evtchn: convert domain event lock to an r/w one To: Julien Grall 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> <1f3571eb-5aec-e76e-0b61-2602356fb436@xen.org> From: Jan Beulich Message-ID: <099b99bc-c544-0aa8-c3b4-4871ef618e4a@suse.com> Date: Tue, 22 Dec 2020 10:46:54 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <1f3571eb-5aec-e76e-0b61-2602356fb436@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 21.12.2020 18:45, Julien Grall wrote: > 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. Well, okay. Three considerations though: 1) Neither evtchn_status() nor domain_dump_evtchn_info() appear to have a real need to acquire the per-domain lock. They could as well acquire the per-channel ones. (In the latter case this will then also allow inserting the so far missing process_pending_softirqs() call; it shouldn't be made with a lock held.) 2) With the double-locking changed and with 1) addressed, there's going to be almost no read_lock() left. hvm_migrate_pirqs() and do_physdev_op()'s PHYSDEVOP_eoi handling, evtchn_move_pirqs(), and hvm_dpci_msi_eoi(). While for these it may still be helpful to be possible to run in parallel, I then nevertheless wonder whether the change as a whole is still worthwhile. 3) With the per-channel double locking and with 1) addressed I can't really see the need for the double per-domain locking in evtchn_bind_interdomain() and evtchn_close(). The write lock is needed for the domain allocating a new port or freeing one. But why is there any need for holding the remote domain's lock, when its side of the channel gets guarded by the per-channel lock anyway? Granted the per-channel locks may then need acquiring a little earlier, before checking the remote channel's state. But this shouldn't be an issue. I guess I'll make addressing 1) and 3) prereq patches to this one, unless I learn of reasons why things need to remain the way they are. Jan