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    SubjectRe: [PATCH 3/3] of/gpio: Introduce of_put_gpio(), add ref counting for OF GPIO chips
    On Tue, Feb 9, 2010 at 12:14 PM, Anton Vorontsov
    <> wrote:
    > On Tue, Feb 09, 2010 at 10:28:15AM -0700, Grant Likely wrote:
    > [...]
    >> Rather than having a lock at the device tree data pointer level which
    >> mixes usage with potentially many other drivers; wouldn't it make more
    >> sense to use a mutex at the of_gc subsystem context?
    > I don't think so.
    > of_gc = np->data;
    > lock(of_gc); (or lock(devtree))
    > <do something with of_gc>
    > doesn't provide us what we need, i.e. it doesn't guarantee that
    > np->data (of_gc) is still alive.
    > And here:
    > lock(np->data); (or lock(devtree))
    > of_gc = np->data;
    > lock(of_gc);
    > <do something with of_gc>
    > The second lock becomes useless (unless you also refcount np->data
    > usage and can drop the devtree/np->data lock, and grab some other
    > kind of lock, e.g. mutex, but this is silly).

    Okay, I'm convinced now. The model is wrong. struct of_gc does need
    to be a member of struct gpio_chip and conditionally compiled against
    CONFIG_OF_GPIO. This locking requirement is just too plain ugly, and
    dereferencing the np->data pointer in this way is dangerous (what if
    something that is not struct of_gc is stored there).

    Put of_gc into struct gpio_chip, and I'll completely support that approach.


    Grant Likely, B.Sc., P.Eng.
    Secret Lab Technologies Ltd.

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