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    SubjectRe: klists and struct device semaphores
    On Mon, 28 Mar 2005, Patrick Mochel wrote:

    > It's important when removing a containing object's knode from the list
    > when that object is about to be freed. This happens during both device and
    > driver unregistration. In most cases, the removal operation will return
    > immediately. When it doesn't, it means another thread is using that
    > particular knode, which means its imperative that the containing object
    > not be freed.
    > Do you have suggestions about an alternative (with code)?

    Here's something a little better than pseudocode but not as good as a
    patch... :-)

    Consider adding to struct klist two new fields:

    int k_offset_to_containers_kref;
    void (*k_containers_kref_release)(struct kref *);

    To fill the first field in correctly requires that klist creation use a
    macro; the details are unimportant. What is important is that during
    klist_node_init you add:

    struct kref *containers_kref = (struct kref *) ((void *) n +


    and in klist_release you add:

    struct kref *containers_kref = (struct kref *) ((void *) n +

    kref_put(containers_kref, n->n_klist->k_containers_kref_release);

    (Actually this conflicts with my earlier suggestion about removing
    n->n_klist. Oh well... nothing's perfect.)

    In fact the kref_put() should take the place of the call to complete().
    This scheme assumes that the container object does contain a kref, but
    this is true for all the containers in the driver model.

    > Good point. It's trivial to add an atomic flag (.n_attached) which is
    > checked during an iteration. This can also be used for the
    > klist_node_attached() function that I posted a few days ago (and you may
    > have missed).

    There's no need for the flag to be atomic, since it's only altered while
    the klist's lock is held.

    > It's assumed that the controlling subsystem will handle lifetime-based
    > reference counting for the containing objects. If you can point me to a
    > potential usage where this assumption would get us into trouble, I'd be
    > interested in trying to work arond this.

    It's not that you get into trouble; it's that you're forced to wait for
    klist_node.n_removed when you shouldn't have to. To put it another way,
    one of the big advantages of the refcounting approach is that it allows
    you to avoid blocking on deallocations -- the deallocation happens
    automatically when the last reference is dropped. Your code loses this
    advantage; it's not the refcounting way.

    If you replace the struct completion with the offset to the container's
    kref and make the klist_node hold a reference to its container, as
    described above, then this unpleasantness can go away.

    Alan Stern

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