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=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 10349C433DB for ; Mon, 8 Feb 2021 09:49:02 +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 9CC2E64E9A for ; Mon, 8 Feb 2021 09:49:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9CC2E64E9A 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.82674.152672 (Exim 4.92) (envelope-from ) id 1l93A3-0002eP-Uz; Mon, 08 Feb 2021 09:48:51 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 82674.152672; Mon, 08 Feb 2021 09:48:51 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1l93A3-0002eI-S3; Mon, 08 Feb 2021 09:48:51 +0000 Received: by outflank-mailman (input) for mailman id 82674; Mon, 08 Feb 2021 09:48:50 +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 1l93A2-0002eD-GN for xen-devel@lists.xenproject.org; Mon, 08 Feb 2021 09:48:50 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 2a4aac4d-1c80-4c40-9132-7dc81b48bd33; Mon, 08 Feb 2021 09:48:49 +0000 (UTC) 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) 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: 2a4aac4d-1c80-4c40-9132-7dc81b48bd33 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= 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 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