Everyone has convinced me that there's no point in making usbdev->serialize an rw-semaphore, so that's out. A big relief to some people, no doubt :-)
I still have a problem with marking devices as soon as we know they are unplugged. Even using David's scheme where this is done using a separate unprotected flag in struct usb_device, I can't see any way to make it work without introducing a new lock.
Hmm, I seem to recall hearing more approval of the variant that uses a spinlock to protect the flag during dev->state transitions.
In fact, the requirements you list below are a great match for a spinlock: virtually no contention (connect and disconnect are rare), no heavy work to be done (no kmalloc etc), clearly defined scope (protects a data structure tree, for one brief shining moment).
So the disconnect would be "grab the lock, walk the subtree being disconnected and mark everything disconnected, down_trylock() to get dev->serialize and if it succeeds, mark it NOTATTACHED". And then schedule khubd disconnect processing for those devices, to unbind all the drivers and destroy all the devices.
- Dave
The problem is that when the "unplugged" flag is set for a hub, it must be set recursively for all the children as well. This requires reading through the list of the hub's children, but it takes place asynchronously with respect to other tasks that may be updating the list. It's not enough to say those other tasks should check the hub's unplugged flag before they update anything; setting and checking a flag does not constitute a true synchronization operation. Either a spinlock or a semaphore is required.
Basically the new lock would have to be held while adding or removing a device from a hub's list of children and while the recursive set-the-unplugged-flag takes place. This doesn't seem likely to create a lot of contention, but still, if anyone can think of a way to do this without a lock I would be pleased to hear it.
Alan Stern
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