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    SubjectRe: [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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