linux-kernel - Re: [PATCH v2] Staging: dgnc: release the lock before testing for nullity

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Date:	Wed, 18 Mar 2015 16:54:02 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Quentin Lambert <lambert.quentin@...il.com>
Cc:	Lidza Louina <lidza.louina@...il.com>, devel@...verdev.osuosl.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	driverdev-devel@...uxdriverproject.org,
	kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Staging: dgnc: release the lock before testing for
 nullity

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.

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.

This kind of change is a bit dangerous though so it requires testing.

regards,
dan carpenter
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