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    SubjectRe: [PATCH RFD] alternative kobject release wait mechanism
    Hello,

    Alan Stern wrote:
    > On Thu, 19 Apr 2007, Tejun Heo wrote:
    >
    >> The goal of immediate-disconnect is to remove such lingering reference
    >> counts so that device_unregister() or driver detach puts the last
    >> reference count.
    >
    > Yes, I understand. If you had immediate-disconnect then you wouldn't need
    > device_unregister_wait(). In fact, you wouldn't need any reference counts
    > at all. It would be guaranteed that when the unregister call returned,
    > all references would be gone.
    >
    >> You tell a higher layer that a device is going away, on return from the
    >> function, that layer isn't gonna access the device anymore.
    >
    > No, no. You tell somebody (it might be a higher layer, it might be a
    > lower layer, or it might be a same-height layer -- doesn't matter) that a
    > device is going away.

    Yeap, right. I higher, lower, same, whatever. I was using the term as
    drivers usually register to upper layers.

    > On return from the function, that layer isn't going
    > to access the device any more, _nor_ will anyone else who has obtained a
    > reference from that layer. This last clause is very important.

    Agreed. That layer is responsible for managing lingering objects and
    telling its users that the device is a zombie now.

    >> I don't think this is gonna be too difficult to do. I think I can
    >> convert block layer and IDE/SCSI drivers without too much problem.
    >> Dunno much about other layers tho.
    >
    > You have to convert more than layers (or core subsystems). You also have
    > to audit and convert drivers. It will be tremendously difficult to do.

    I definitely can be mis-assessing the problem. I'll first give a shot
    at the block/SCSI layer. How about that?

    > You did misunderstand. Here's what I was talking about:
    >
    > Driver A:
    > ---------
    > unregister_device(dev);
    >
    > /* inside the driver core */
    > down(&dev->sem);
    > if (dev->driver)
    > dev->driver->remove(dev);
    > up(&dev->sem);
    > device_put(dev); /* or device_put_wait */
    >
    >
    > Driver B:
    > ---------
    > void remove(struct device *dev)
    > {
    > struct my_device *mydev = dev_get_drvdata(dev);
    >
    > mydev->gone = 1;
    > kref_put(&mydev->kref, my_device_release);
    > }
    >
    >
    > Driver B's kernel thread:
    > -------------------------
    > kref_get(&mydev->kref);
    > down(&mydev->dev.sem);
    > if (mydev->gone)
    > goto finished;
    > ...
    > finished:
    > up(&mydev->dev.sem);
    > kref_put(&mydev->kref, my_device_release);
    >
    > Consider what happens if the kernel thread blocks on its down() while the
    > remove() method is running. It will be impossible for Driver B to
    > eliminate the reference to dev held by mydev and by the down() routine.
    >
    > In short, Driver B _can't_ provide an immediate detach. Not unless
    > someone figures out a way to cancel a blocked down(). And do the same
    > thing for other blocking primitives.

    Ah.. I see. You're right in that driver B cannot wait for disconnect in
    its remove routine in the above code but using a separate mutex to
    protect ->gone should do the trick, so I don't think the above case is a
    big problem. It's a pretty specific case which is easy to spot and update.

    Thanks.

    --
    tejun
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