Hard spinlocks manipulate the CPU interrupt mask, without affecting
the kernel preemption state in locking/unlocking operations.

This type of spinlock is useful for implementing a critical section to
serialize concurrent accesses from both in-band and out-of-band
contexts, i.e. from root and head stages.

Hard spinlocks exclusively depend on the pre-existing arch-specific
bits which implement regular spinlocks. They can be seen as basic
spinlocks still affecting the CPU's interrupt state when all other
spinlock types only deal with the virtual interrupt flag managed by
the pipeline core - i.e. only disable interrupts for the regular
in-band kernel activity.