From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756664AbbCRN7Q (ORCPT ); Wed, 18 Mar 2015 09:59:16 -0400 Received: from mail-wg0-f43.google.com ([74.125.82.43]:34614 "EHLO mail-wg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756439AbbCRN7L (ORCPT ); Wed, 18 Mar 2015 09:59:11 -0400 Message-ID: <550984AE.5030802@gmail.com> Date: Wed, 18 Mar 2015 14:59:10 +0100 From: Quentin Lambert User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 MIME-Version: 1.0 To: Dan Carpenter CC: Lidza Louina , devel@driverdev.osuosl.org, Greg Kroah-Hartman , driverdev-devel@linuxdriverproject.org, kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] Staging: dgnc: release the lock before testing for nullity References: <1426684868-24402-1-git-send-email-lambert.quentin@gmail.com> <20150318133651.GS10964@mwanda> <550980E5.8080001@gmail.com> <20150318135402.GU16501@mwanda> In-Reply-To: <20150318135402.GU16501@mwanda> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/03/2015 14:54, Dan Carpenter wrote: > On Wed, Mar 18, 2015 at 02:43:01PM +0100, Quentin Lambert wrote: >> >> On 18/03/2015 14:36, Dan Carpenter wrote: >>> This changelog still doesn't make sense so I took a look at the code. >>> >>> tty_ldisc_deref() is an unlock function. So this is a lock ordering >>> bug. What makes you think the original ordering was correct? Who >>> reported this bug? What are the effects of this bug? >> I was the one who introduced the ordering change in the first place. >> I am just trying to fix it because although nobody complained I am not >> sure of the impact and restoring the previous control flow seems to be the >> right thing to do. > Your changelog should tell me this stuff. Should I send a third version then? > The original code is wrong. We take "spin_lock_irqsave(&ch->ch_lock, > flags);" before we do "ld = tty_ldisc_ref(tp);" so we should deref > before we unlock. > > It's normally: > > lock_outer(); > lock_inner(); > unlock_inner(); > unlock_outer(); > > On the success path we unlock first then deref and that is a mistake. I didn't know that thank you. > This kind of change is a bit dangerous though so it requires testing. Ok, should I act on that? What do you advice? > regards, > dan carpenter