Revisiting the kernel's preemption models (part 1)

It sounds like the reason it isn't possible is that code is compiled differently, e.g. "even most spinlocks become preemptible" - in fact they are no longer spinlocks at all, but relatively normal locks where waiting code sleeps (yields to another process) instead of spinning: https://wiki.linuxfoundation.org/realtime/documentation/t...

So you can't really convert this in a running system, because locks can be held at any time. You'd have to acquire the spinlock (which can block), then turn it into a sleeping lock, then figure out who's been spinning on the spinlock while you held it and somehow convert them. For a use case like going into battle, even if this were technically possible, having the mode switch require acquiring every lock in the system isn't acceptable because it could take indefinitely long. (And even if it weren't for this, there would be an ongoing performance cost in both modes in order to support checking what mode you're in and so forth, meaning that performance will never be as good as a pure real-time or pure non-real-time kernel.)