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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 44A86C169C4 for ; Thu, 31 Jan 2019 13:49:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 203D62085B for ; Thu, 31 Jan 2019 13:49:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733012AbfAaNtw (ORCPT ); Thu, 31 Jan 2019 08:49:52 -0500 Received: from relay.sw.ru ([185.231.240.75]:46226 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732534AbfAaNtw (ORCPT ); Thu, 31 Jan 2019 08:49:52 -0500 Received: from [172.16.25.169] by relay.sw.ru with esmtp (Exim 4.91) (envelope-from ) id 1gpCio-0002Oq-9i; Thu, 31 Jan 2019 16:49:38 +0300 Subject: Re: [PATCH] net: check negative value for signed refcnt To: alexandre.besnard@softathome.com, davem@davemloft.net, ecree@solarflare.com, jiri@mellanox.com, petrm@mellanox.com, alexander.h.duyck@intel.com, amritha.nambiar@intel.com, lirongqing@baidu.com Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20190131132008.23161-1-alexandre.besnard@softathome.com> From: Kirill Tkhai Message-ID: <8b654b17-f8b4-0c89-26b5-311aeb703f6d@virtuozzo.com> Date: Thu, 31 Jan 2019 16:49:38 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190131132008.23161-1-alexandre.besnard@softathome.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Alexandre, On 31.01.2019 16:20, alexandre.besnard@softathome.com wrote: > From: Alexandre Besnard > > Device remaining references counter is get as a signed integer. > > When unregistering network devices, the loop waiting for this counter > to decrement tests the 0 strict equality. Thus if an error occurs and > two references are given back by a protocol, we are stuck in the loop > forever, with a -1 value. > > Robustness is added by checking a negative value: the device is then > considered free of references, and a warning is issued (it should not > happen, one should check that behavior) > > Signed-off-by: Alexandre Besnard > --- > net/core/dev.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/net/core/dev.c b/net/core/dev.c > index ddc551f..e4190ae 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -8687,6 +8687,11 @@ static void netdev_wait_allrefs(struct net_device *dev) > refcnt = netdev_refcnt_read(dev); > > while (refcnt != 0) { > + if (refcnt < 0) { > + pr_warn("Device %s refcnt negative: device considered free, but it should not happen\n", > + dev->name); > + break; > + } 1)I don't think this is a good approach. Negative value does not guarantee there is just a double put of device reference. Negative value is an indicator something goes wrong, and we definitely should not free device memory in this case. 2)Not related to your patch -- it looks like we have problem in existing code with this netdev_refcnt_read(). It does not imply a memory ordering or some guarantees about reading percpu values. For example, in generic code struct percpu_ref switches a counter into atomic mode before it checks for the last reference. But there is nothing in netdev_refcnt_read().