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 C866BC4338F for ; Tue, 24 Aug 2021 01:02:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AFCBA61245 for ; Tue, 24 Aug 2021 01:02:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234887AbhHXBCw (ORCPT ); Mon, 23 Aug 2021 21:02:52 -0400 Received: from mail.kernel.org ([198.145.29.99]:48386 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233870AbhHXA5N (ORCPT ); Mon, 23 Aug 2021 20:57:13 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5954A6154B; Tue, 24 Aug 2021 00:55:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1629766541; bh=abhwVH7jxZvJC/QUmFVQU1Z3zBylCv/M+6tQw5EQd+I=; h=From:To:Cc:Subject:Date:From; b=jCInE6RiTLJS2TuexDftke6meOtkJV9KDII7AKEaiNz+/uhMRZscASNyClyw+W5aG U1uLieF92EfL+Pjc09RqZSdqViy0R9lObVYKnCjJ6KsEM+7Q/nVMmMkj2P1fQRDyLh CMdJGmO7FCNAGjuLH15Xu3nJC2dD6EtSJC/WvC0RcW5QX3YLUsFEkBEm9RNw0p6G9e nz9mNc/DKKX1lwcRtaUuDWEVoHsOL4JvLQgX1UZ8cjxF9U0hvKEuukveS7DsDF8F4/ VMoTPsbo+po3C2RNSaAvplgGxxmgQ9iQ5Vll0Tz5f3SXSAbMk6gcPuCa4P52pgasEy EFZa11gTm5tiQ== 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 4.9 1/3] virtio: Improve vq->broken access to avoid any compiler optimization Date: Mon, 23 Aug 2021 20:55:37 -0400 Message-Id: <20210824005539.631820-1-sashal@kernel.org> X-Mailer: git-send-email 2.30.2 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 5cad9f41c238..cf7eccfe3469 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -1150,7 +1150,7 @@ 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); @@ -1164,7 +1164,9 @@ 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