In Rtlinux all threads runs in the kernel address space,that is the reason why they can accede to all the variables and functions of the kernel and calls to the linux sistems are forbidden, also run supervisor mode , what means that can execute any processor instruction and they have access to all the I/O ports . This allows too avoid the overhead of protection level changes, improving performances and making switching easier. However,when such protection is disabled, a possible bug can appear hanging the whole system up.
As Yodaiken assert the RT kernel does not request memory, share spin-locks, or synchronize on any data structures except in tightly controlled situations, being the functional design the quite robust enough to give a suitable real time operation.