The WDF guys needed to solve the blocking-call thing of course; their approach was to say that I/O request packets had to be either completed within a driver callback or else placed in a queue for later retrieval in response to an event or timer; placing it in an internal queue would release the device or instance-wide lock. Busy-waiting or sleeping inside a callback was always an error. It's troubling for this purpose that Linux's model of in-progress kernel calls is very much tied to the C stack, but it seems like it wouldn't be too difficult to remodel using aio as the standard and synchronous calls as a simple wrapper atop that which takes place outside the driver framework's locking scheme.
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