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=-8.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SIGNED_OFF_BY,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 895E9C433DF for ; Tue, 13 Oct 2020 14:26:40 +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 0B3B62481E for ; Tue, 13 Oct 2020 14:26:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="k6gYdhVe" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0B3B62481E Authentication-Results: mail.kernel.org; dmarc=fail (p=none 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.6254.16666 (Exim 4.92) (envelope-from ) id 1kSLFv-0001na-G2; Tue, 13 Oct 2020 14:26:23 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 6254.16666; Tue, 13 Oct 2020 14:26:23 +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" Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kSLFv-0001nT-BM; Tue, 13 Oct 2020 14:26:23 +0000 Received: by outflank-mailman (input) for mailman id 6254; Tue, 13 Oct 2020 14:26:21 +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 1kSLFt-0001nO-CR for xen-devel@lists.xenproject.org; Tue, 13 Oct 2020 14:26:21 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 655d70f9-c66a-487a-aa8d-ada1037ddda6; Tue, 13 Oct 2020 14:26:19 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A038AAC7D; Tue, 13 Oct 2020 14:26:18 +0000 (UTC) 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 1kSLFt-0001nO-CR for xen-devel@lists.xenproject.org; Tue, 13 Oct 2020 14:26:21 +0000 X-Inumbo-ID: 655d70f9-c66a-487a-aa8d-ada1037ddda6 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 655d70f9-c66a-487a-aa8d-ada1037ddda6; Tue, 13 Oct 2020 14:26:19 +0000 (UTC) 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=1602599178; h=from:from:reply-to:subject:subject: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=RZcX6dU+VQOJj1Sh5V7A54bk1gbLeruICYDTGZZO0o8=; b=k6gYdhVea2Vqy2kNjhtoS1P34tC4dvQiX2FiK4YHvDpcMkVPolvwsp5mnHlbtuPsFOlahZ w1c7tmlHvTO00nKFmaylkQIiGQdg1ojb4ZjSpUy7ylxPnOovCU04nT/BLtnSrGSS5FP+Cb F9I6vDeKYxjBp5vLtcB4Y1DOqq2x1Ek= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id A038AAC7D; Tue, 13 Oct 2020 14:26:18 +0000 (UTC) Subject: Re: [PATCH v2 1/2] xen/events: access last_priority and last_vcpu_id together To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , Julien Grall Cc: xen-devel@lists.xenproject.org, Andrew Cooper , George Dunlap , Ian Jackson , Stefano Stabellini , Wei Liu References: <20201012092740.1617-1-jgross@suse.com> <20201012092740.1617-2-jgross@suse.com> <9485004c-b739-5590-202b-c8e6f84e5e54@suse.com> <821a77d3-7e37-d1d2-d904-94db0177893a@suse.com> From: Jan Beulich Message-ID: <350a5738-b239-e36b-59aa-05b8f86648b8@suse.com> Date: Tue, 13 Oct 2020 16:26:15 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <821a77d3-7e37-d1d2-d904-94db0177893a@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 13.10.2020 16:20, Jürgen Groß wrote: > On 13.10.20 15:58, Jan Beulich wrote: >> On 12.10.2020 11:27, Juergen Gross wrote: >>> The queue for a fifo event is depending on the vcpu_id and the >>> priority of the event. When sending an event it might happen the >>> event needs to change queues and the old queue needs to be kept for >>> keeping the links between queue elements intact. For this purpose >>> the event channel contains last_priority and last_vcpu_id values >>> elements for being able to identify the old queue. >>> >>> In order to avoid races always access last_priority and last_vcpu_id >>> with a single atomic operation avoiding any inconsistencies. >>> >>> Signed-off-by: Juergen Gross >> >> I seem to vaguely recall that at the time this seemingly racy >> access was done on purpose by David. Did you go look at the >> old commits to understand whether there really is a race which >> can't be tolerated within the spec? > > At least the comments in the code tell us that the race regarding > the writing of priority (not last_priority) is acceptable. Ah, then it was comments. I knew I read something to this effect somewhere, recently. > Especially Julien was rather worried by the current situation. In > case you can convince him the current handling is fine, we can > easily drop this patch. Julien, in the light of the above - can you clarify the specific concerns you (still) have? >>> --- a/xen/include/xen/sched.h >>> +++ b/xen/include/xen/sched.h >>> @@ -114,8 +114,7 @@ struct evtchn >>> u16 virq; /* state == ECS_VIRQ */ >>> } u; >>> u8 priority; >>> - u8 last_priority; >>> - u16 last_vcpu_id; >>> + u32 fifo_lastq; /* Data for fifo events identifying last queue. */ >> >> This grows struct evtchn's size on at least 32-bit Arm. I'd >> like to suggest including "priority" in the union, and call the >> new field simply "fifo" or some such. > > This will add quite some complexity as suddenly all writes to the > union will need to be made through a cmpxchg() scheme. > > Regarding the growth: struct evtchn is aligned to 64 bytes. So there > is no growth at all, as the size will not be larger than those 64 > bytes. Oh, I didn't spot this attribute, which I consider at least suspicious. Without XSM I'm getting the impression that on 32-bit Arm the structure's size would be 32 bytes or less without it, so it looks as if it shouldn't be there unconditionally. Jan