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

From: Dan Carpenter
Date: Wed Mar 18 2015 - 09:54:26 EST

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:


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.

dan carpenter
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