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=-17.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 49385C433DB for ; Mon, 8 Feb 2021 12:16:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0701064D79 for ; Mon, 8 Feb 2021 12:16:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231216AbhBHMPy (ORCPT ); Mon, 8 Feb 2021 07:15:54 -0500 Received: from mx2.suse.de ([195.135.220.15]:36812 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232854AbhBHLz0 (ORCPT ); Mon, 8 Feb 2021 06:55:26 -0500 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=1612785280; 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=Lzj0M0ODVg7irUOYnOdjiNxBrgsVBdOtnhI9h2vRceY=; b=GqVgYoTCsAEXTwhIkrKe4y4fnUSBbqbfH23KdRhYyjhWoTUQxU2YjCKiSMxHC0q7DR7pAj BjGXzvTTXLSBHZOJetheHO6MsUo02OPb8NcEhBwEO5KrdYtca0SdCU0+ZoRAlBl5pJ6eUr VBFfB0utZwpC7HNrBYRpT1Y4P/IPkW8= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 400A8AEC2; Mon, 8 Feb 2021 11:54:40 +0000 (UTC) Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Cc: Boris Ostrovsky , Stefano Stabellini , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org References: <20210206104932.29064-1-jgross@suse.com> <20210206104932.29064-8-jgross@suse.com> <72334160-cffe-2d8a-23b7-2ea9ab1d803a@suse.com> <626f500a-494a-0141-7bf3-94fb86b47ed4@suse.com> From: Jan Beulich Message-ID: Date: Mon, 8 Feb 2021 12:54:39 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08.02.2021 11:59, Jürgen Groß wrote: > On 08.02.21 11:51, Jan Beulich wrote: >> On 08.02.2021 11:41, Jürgen Groß wrote: >>> On 08.02.21 10:48, Jan Beulich wrote: >>>> On 06.02.2021 11:49, Juergen Gross wrote: >>>>> In evtchn_read() use READ_ONCE() for reading the producer index in >>>>> order to avoid the compiler generating multiple accesses. >>>>> >>>>> Signed-off-by: Juergen Gross >>>>> --- >>>>> drivers/xen/evtchn.c | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c >>>>> index 421382c73d88..f6b199b597bf 100644 >>>>> --- a/drivers/xen/evtchn.c >>>>> +++ b/drivers/xen/evtchn.c >>>>> @@ -211,7 +211,7 @@ static ssize_t evtchn_read(struct file *file, char __user *buf, >>>>> goto unlock_out; >>>>> >>>>> c = u->ring_cons; >>>>> - p = u->ring_prod; >>>>> + p = READ_ONCE(u->ring_prod); >>>>> if (c != p) >>>>> break; >>>> >>>> Why only here and not also in >>>> >>>> rc = wait_event_interruptible(u->evtchn_wait, >>>> u->ring_cons != u->ring_prod); >>>> >>>> or in evtchn_poll()? I understand it's not needed when >>>> ring_prod_lock is held, but that's not the case in the two >>>> afaics named places. Plus isn't the same then true for >>>> ring_cons and ring_cons_mutex, i.e. aren't the two named >>>> places plus evtchn_interrupt() also in need of READ_ONCE() >>>> for ring_cons? >>> >>> The problem solved here is the further processing using "p" multiple >>> times. p must not be silently replaced with u->ring_prod by the >>> compiler, so I probably should reword the commit message to say: >>> >>> ... in order to not allow the compiler to refetch p. >> >> I still wouldn't understand the change (and the lack of >> further changes) then: The first further use of p is >> outside the loop, alongside one of c. IOW why would c >> then not need treating the same as p? > > Its value wouldn't change, as ring_cons is being modified only at > the bottom of this function, and nowhere else (apart from the reset > case, but this can't run concurrently due to ring_cons_mutex). > >> I also still don't see the difference between latching a >> value into a local variable vs a "freestanding" access - >> neither are guaranteed to result in exactly one memory >> access afaict. > > READ_ONCE() is using a pointer to volatile, so any refetching by > the compiler would be a bug. Of course, but this wasn't my point. I was contrasting c = u->ring_cons; p = u->ring_prod; which you change with rc = wait_event_interruptible(u->evtchn_wait, u->ring_cons != u->ring_prod); which you leave alone. Jan