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 1FC89C432C3 for ; Mon, 8 Feb 2021 10:00:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DC83C64E56 for ; Mon, 8 Feb 2021 10:00:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232029AbhBHKAH (ORCPT ); Mon, 8 Feb 2021 05:00:07 -0500 Received: from mx2.suse.de ([195.135.220.15]:34018 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232021AbhBHJte (ORCPT ); Mon, 8 Feb 2021 04:49:34 -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=1612777728; 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=3NhcpUvwMgEF2rRU4mx1o9wN8tYHo5OlqhAF8sWXq9M=; b=cumVrgkaunXuO6f9PY170PKriCorRoIVGi2zdDkYcDguu3RJ7sDt6XroUa4+2Bpkdhv2hs BieJgAhpprvHACzZ/tyu106MaAMLcLVdtFAhmKhkINjYZ1TqUQilo3ICF25vqpJR76GEHE u5I8/zp7ei0cTHCwYx+RXxe+5xOs1mY= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id D20D2AC6E; Mon, 8 Feb 2021 09:48:48 +0000 (UTC) Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once To: Juergen Gross 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> From: Jan Beulich Message-ID: <72334160-cffe-2d8a-23b7-2ea9ab1d803a@suse.com> Date: Mon, 8 Feb 2021 10:48:48 +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: <20210206104932.29064-8-jgross@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Jan