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=-19.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT 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 7960EC4320A for ; Wed, 1 Sep 2021 12:31:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C5E1610CA for ; Wed, 1 Sep 2021 12:31:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244959AbhIAMcD (ORCPT ); Wed, 1 Sep 2021 08:32:03 -0400 Received: from mail.kernel.org ([198.145.29.99]:32780 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244831AbhIAMbW (ORCPT ); Wed, 1 Sep 2021 08:31:22 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 6B718610A4; Wed, 1 Sep 2021 12:30:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1630499426; bh=nlhbgY7JlbZUUHfACAanG1o0XxU/k6vH9k9y4N0S8zY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WFRNqXg+Kws3msJ2trM7HiRWESm3Tkd7vZnWMuFygO1XtAYXnxBOS7LGcEPyojIA+ ggqKrp6BUMs9egQ+KrpJb93kikoV3VMP+iCT/ZiNt19uFHItmXGvb2mSdohRhB0iBs LVQI1ITbwANsOtFaviNJhAk1WMZwQL1BxR6PuwI8= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Parav Pandit , "Michael S. Tsirkin" , Sasha Levin Subject: [PATCH 4.19 21/33] virtio: Improve vq->broken access to avoid any compiler optimization Date: Wed, 1 Sep 2021 14:28:10 +0200 Message-Id: <20210901122251.490479445@linuxfoundation.org> X-Mailer: git-send-email 2.33.0 In-Reply-To: <20210901122250.752620302@linuxfoundation.org> References: <20210901122250.752620302@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Parav Pandit [ Upstream commit 60f0779862e4ab943810187752c462e85f5fa371 ] Currently vq->broken field is read by virtqueue_is_broken() in busy loop in one context by virtnet_send_command(). vq->broken is set to true in other process context by virtio_break_device(). Reader and writer are accessing it without any synchronization. This may lead to a compiler optimization which may result to optimize reading vq->broken only once. Hence, force reading vq->broken on each invocation of virtqueue_is_broken() and also force writing it so that such update is visible to the readers. It is a theoretical fix that isn't yet encountered in the field. Signed-off-by: Parav Pandit Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@nvidia.com Signed-off-by: Michael S. Tsirkin Signed-off-by: Sasha Levin --- drivers/virtio/virtio_ring.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index df7980aef927..0cc0cfd3a3cb 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -1197,7 +1197,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq) { struct vring_virtqueue *vq = to_vvq(_vq); - return vq->broken; + return READ_ONCE(vq->broken); } EXPORT_SYMBOL_GPL(virtqueue_is_broken); @@ -1211,7 +1211,9 @@ void virtio_break_device(struct virtio_device *dev) list_for_each_entry(_vq, &dev->vqs, list) { struct vring_virtqueue *vq = to_vvq(_vq); - vq->broken = true; + + /* Pairs with READ_ONCE() in virtqueue_is_broken(). */ + WRITE_ONCE(vq->broken, true); } } EXPORT_SYMBOL_GPL(virtio_break_device); -- 2.30.2