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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 196FFC43219 for ; Mon, 29 Apr 2019 18:48:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DF8B62067D for ; Mon, 29 Apr 2019 18:48:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729093AbfD2SsX (ORCPT ); Mon, 29 Apr 2019 14:48:23 -0400 Received: from mx2.suse.de ([195.135.220.15]:42986 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1729023AbfD2SsW (ORCPT ); Mon, 29 Apr 2019 14:48:22 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 4FE4AAD7B; Mon, 29 Apr 2019 18:48:21 +0000 (UTC) Message-ID: <1556563688.20085.31.camel@suse.com> Subject: Re: [PATCH] usbnet: fix kernel crash after disconnect From: Oliver Neukum To: Jan =?ISO-8859-1?Q?Kl=F6tzke?= , David Miller Cc: Jan.Kloetzke@preh.de, linux-usb@vger.kernel.org, netdev@vger.kernel.org Date: Mon, 29 Apr 2019 20:48:08 +0200 In-Reply-To: <20190419071752.GG1084@tuxedo> References: <20190417091849.7475-1-Jan.Kloetzke@preh.de> <1555569464.7835.4.camel@suse.com> <1555574578.4173.215.camel@preh.de> <20190418.163544.2153438649838575906.davem@davemloft.net> <20190419071752.GG1084@tuxedo> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.6 Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fr, 2019-04-19 at 09:17 +0200, Jan Klötzke wrote: > Hi David, > > On Thu, Apr 18, 2019 at 04:35:44PM -0700, David Miller wrote: > > From: Kloetzke Jan > > Date: Thu, 18 Apr 2019 08:02:59 +0000 > > > > > I think this assumption is not correct. As far as I understand the > > > networking code it is still possible that the ndo_start_xmit callback > > > is called while ndo_stop is running and even after ndo_stop has > > > returned. You can only be sure after unregister_netdev() has returned. > > > Maybe some networking folks can comment on that. > > > > The kernel loops over the devices being unregistered, and first it clears > > the __LINK_STATE_START on all of them, then it invokes ->ndo_stop() on > > all of them. > > > > __LINK_STATE_START controls what netif_running() returns. > > > > All calls to ->ndo_start_xmit() are guarded by netif_running() checks. > > > > So when ndo_stop is invoked you should get no more ndo_start_xmit > > invocations on that device. Otherwise how could you shut down DMA > > resources and turn off the TX engine properly? > > But you could still race with another CPU that is past the > netif_running() check, can you? So the driver has to make sure that it > gracefully handles concurrent ->ndo_start_xmit() and ->ndo_stop() calls. Looking at dev_direct_xmit(struct sk_buff *skb, u16 queue_id) this indeed seems possible. But the documentation says that it is not. Dave? > Or are there any locks/barriers involved that make sure all > ->ndo_start_xmit() calls have returned before invoking ->ndo_stop()? Jan, could you make versio of your patch that gives a WARNing if this race triggers? Regards Oliver