That is because you do not need to clear the field at the end of a critical section. It contains the contiguous instruction range where it fires so there is no problem with leaving it active forever unless you have another critical section where you want to use it.

No explicit memory barrier is required anywhere as the value is only read in supervisor mode and a privilege switch implicitly issues a LS-LS barrier on all major architectures. Even if you did not want to rely on that, you would only need a single S-LS barrier when you store the control structure the very first time.