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=-5.5 required=3.0 tests=BAYES_00, 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 7E06EC433E1 for ; Wed, 15 Jul 2020 13:51:41 +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 516B220657 for ; Wed, 15 Jul 2020 13:51:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 516B220657 Authentication-Results: mail.kernel.org; dmarc=none (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 localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jvhoe-0004QA-J6; Wed, 15 Jul 2020 13:51:20 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jvhoc-0004Q2-Un for xen-devel@lists.xenproject.org; Wed, 15 Jul 2020 13:51:18 +0000 X-Inumbo-ID: 41745504-c6a2-11ea-bca7-bc764e2007e4 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 41745504-c6a2-11ea-bca7-bc764e2007e4; Wed, 15 Jul 2020 13:51:17 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 3A4CFAC53; Wed, 15 Jul 2020 13:51:20 +0000 (UTC) Subject: Re: [PATCH v3 1/2] x86: restore pv_rtc_handler() invocation To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <5426dd6f-50cd-dc23-5c6b-0ab631d98d38@suse.com> <7dd4b668-06ca-807a-9cc1-77430b2376a8@suse.com> <20200715121347.GY7191@Air-de-Roger> <2b9de0fd-5973-ed66-868c-ffadca83edf3@suse.com> <20200715133217.GZ7191@Air-de-Roger> From: Jan Beulich Message-ID: Date: Wed, 15 Jul 2020 15:51:17 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200715133217.GZ7191@Air-de-Roger> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: "xen-devel@lists.xenproject.org" , Paul Durrant , Wei Liu , Andrew Cooper Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 15.07.2020 15:32, Roger Pau Monné wrote: > On Wed, Jul 15, 2020 at 02:36:49PM +0200, Jan Beulich wrote: >> On 15.07.2020 14:13, Roger Pau Monné wrote: >>> On Wed, Jul 15, 2020 at 01:56:47PM +0200, Jan Beulich wrote: >>>> @@ -1160,6 +1162,14 @@ void rtc_guest_write(unsigned int port, >>>> case RTC_PORT(1): >>>> if ( !ioports_access_permitted(currd, RTC_PORT(0), RTC_PORT(1)) ) >>>> break; >>>> + >>>> + spin_lock_irqsave(&rtc_lock, flags); >>>> + hook = pv_rtc_handler; >>>> + spin_unlock_irqrestore(&rtc_lock, flags); >>> >>> Given that clearing the pv_rtc_handler variable in handle_rtc_once is >>> not done while holding the rtc_lock, I'm not sure there's much point >>> in holding the lock here, ie: just doing something like: >>> >>> hook = pv_rtc_handler; >>> if ( hook ) >>> hook(currd->arch.cmos_idx & 0x7f, data); >>> >>> Should be as safe as what you do. >> >> No, the compiler is free to eliminate the local variable and read >> the global one twice (and it may change contents in between) then. >> I could use ACCESS_ONCE() or read_atomic() here, but then it would >> become quite clear that at the same time ... >> >>> We also assume that setting pv_rtc_handler to NULL is an atomic >>> operation. >> >> ... this (which isn't different from what we do elsewhere, and we >> really can't fix everything at the same time) ought to also become >> ACCESS_ONCE() (or write_atomic()). >> >> A compromise might be to use barrier() in place of the locking for >> now ... > > Oh, right. Didn't realize you did it in order to prevent > optimizations. Using the lock seems also quite weird IMO, so I'm not > sure it's much better than just using ACCESS_ONCE (or a barrier). > Anyway, I don't want to delay this any longer, so: > > Reviewed-by: Roger Pau Monné Thanks. > Feel free to change to ACCESS_ONCE or barrier if you think it's > clearer. I did so (also on the writer side), not the least based on guessing what Andrew would presumably have preferred. Jan