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    SubjectRe: [Patch 0/2] sysfs: fix s_active lockdep warning
    On Thu, 4 Feb 2010, Peter Zijlstra wrote:

    > > This doesn't address the fact that we really have multiple device trees
    > > (for example, class devices are handled separately from normal
    > > devices). With the above patch installed, I still get lockdep
    > > violations farther on during boot:
    > <snip lockdep splat>
    > Hmm, so you have multiple interacting trees? I had understood you only
    > had a single device tree.

    The real situation is kind of complicated, and I'm not familiar with
    all the details. But it's certainly true that a driver will want to
    work with (and lock!) multiple struct device's that don't have a
    parent-child relation in the tree. The simplest example is regular
    devices together with class devices, and another might be PCI devices
    together with their "shadow" ACPI devices.

    > So how many trees are there, is that fixed?
    > Does the device know what tree it is going to end up in?

    The driver generally knows, but AFAIK that information is not passed
    back to the driver core. At least, not directly -- you might say that
    it could be deduced from the parent pointer, assuming the core already
    knows all about the parent.

    > If yes, then you can extend the setup_mutex_depth() function to pick a
    > different class stack for each tree.

    Maybe this could be done. But for now perhaps a compromise is in
    order. We could make the switch from semaphores to mutexes while
    avoiding lockdep issues by assigning the device mutexes to a
    "don't-verify" class. Is there such a thing, or could it be added?

    Alan Stern

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