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

[Quentin Lambert]

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

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