On Thu, 30 Jun 2011, Sarah Sharp wrote: > On Thu, Jun 30, 2011 at 05:01:36PM -0400, Alan Stern wrote: > > While iterating through a single bus, all you need to do is lock each > > device as you come to it (including the root hub), using > > usb_lock_device() and usb_unlock_device(). While you hold a device's > > lock, it can't be unregistered and it can't have any children added or > > removed. > > Say I have this tree: > > port 2 > | > HS hub 1 > / \ > HS hub 2 HS device 3 > > If I call usb_lock_device() for HS device 3, and then for HS hub 1, > does that prevent children from being added or removed from HS hub 2? Ack! Don't do that! The device-model locking rules require that locks be acquired going _down_ the tree, never _up_. Therefore you must never call usb_lock_device() for hub 1 while holding the lock for either hub 2 or device 3. To answer your question: The only way to prevent children from being added to hub 2 is to lock hub 2. You can prevent a child from being removed from hub 2 by locking hub 2, the child, or any of the child's descendants. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
