Clarification: Acquiring a mutex when a read write lock is held

[Dheeraj Kandula]

Hey All,
       Can you clarify the following

I have a read write lock locked. Next I want to acquire a mutex when the
read write lock is held. Is this allowed or not. As far as I can understand
from the "FreeBSD Kernel Developers Handbook" this operation is allowed in
FreeBSD9.0, 10.0 and further.

But the read write lock cannot be held while sleeping. Doesn't this
contradict the above statement, as a thread will go to sleep if can't
acquire the mutex when another thread holds the mutex.

Hence what sleep are we referring here. Is this sleep(unbounded sleep) or
the bounded sleep.

Dheeraj