From mboxrd@z Thu Jan 1 00:00:00 1970 From: Quentin Lambert Date: Wed, 18 Mar 2015 13:59:10 +0000 Subject: Re: [PATCH v2] Staging: dgnc: release the lock before testing for nullity Message-Id: <550984AE.5030802@gmail.com> List-Id: 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> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Dan Carpenter Cc: devel@driverdev.osuosl.org, Lidza Louina , driverdev-devel@linuxdriverproject.org, kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org, Greg Kroah-Hartman 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