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=-19.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 D1385C4338F for ; Tue, 24 Aug 2021 01:01:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B5F1B61245 for ; Tue, 24 Aug 2021 01:01:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235183AbhHXBCW (ORCPT ); Mon, 23 Aug 2021 21:02:22 -0400 Received: from mail.kernel.org ([198.145.29.99]:47676 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234909AbhHXA4p (ORCPT ); Mon, 23 Aug 2021 20:56:45 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id A3C8B6141B; Tue, 24 Aug 2021 00:55:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1629766531; bh=zWT3e4gJTxHTTlbmAOYA9XitnOFdGdTFTMYqDai0HYI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=HIDxx0AxOLb1+c+4oh05jjKFTA6vmkhI12UCNhyAzgihscOnwRkkyJhNTOCjb1oAx Hn5CqnFyrbS0G/QZvF1zODgNq8O/ilTw/Z1nyv6TajeuOHuy6Y1ZlBmrNSi5GsZOh2 YSoSgWV7a7ZyQJZgwK/2uje0BFsO2YnS78KoxirdLdyf3pU7GseiVhtCDrDAc17JDX f8VUQSIOaNxE19peoMGetYRagRcHLG/2YhP1TZetNbzhCA1RpRrinkql7pzR54YWb9 tJty+RvKPIkJ8Gy2RZiVHP6GIr/QKZIAj3xZ5srmybVQYrVw0h2XMvkkV+WJeI40Lh 3l8yJIBSToR7Q== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Parav Pandit , "Michael S . Tsirkin" , Sasha Levin , virtualization@lists.linux-foundation.org Subject: [PATCH AUTOSEL 4.14 2/7] virtio: Improve vq->broken access to avoid any compiler optimization Date: Mon, 23 Aug 2021 20:55:23 -0400 Message-Id: <20210824005528.631702-2-sashal@kernel.org> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20210824005528.631702-1-sashal@kernel.org> References: <20210824005528.631702-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@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 51278f8bd3ab..22a4329ed200 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -1198,7 +1198,7 @@ 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); @@ -1212,7 +1212,9 @@ 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