In a multiprocessor, spl*() functions like splimp() or splnet() do block interrupts on the local CPU, but they do not prevent interrupts from occurring on other processors in the system, nor do they prevent other processes on other CPUs from executing code that refers to the same data.
If you are porting a driver from a uniprocessor environment, search for any use of an spl*() function and plan to replace it with effective mutual exclusion locking macros.