Our spinlock implementation allows a particular thread to obtain a lock
multiple times, but release the lock with a single unlock call. Since
we're detecting recursion, we know the lock is already owned by the
current thread in a previous call and must not be released in the
current call. This is really far too dependent on this particular
spinlock implementation, so I've added commented out calls to
THREAD_UNLOCK in the appropriate places. We can activate this code when
spinlock is taught to count each lock operation.
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