Realtime preemption: spinlocks

Spinlocks are a latency problem
Processor "spins" waiting for a lock
Threads holding locks cannot be preempted
Thus: spinlocks can block unrelated processes

The solution: get rid of spinlocks
Create a new type of semaphore
"Mutex"
Includes priority inheritance
Redefine spinlock_t to the new type
Some preprocessor ugliness

As a result
Spinlock contention does not stall processors
Code holding locks can be preempted