Problems In Kernel Space - SMP And Re-Entrancy A process executing a system call might go to sleep, allowing another process to execute this system call. In SMP systems, even if a process does not go to sleep, its code might be executed in parallel in another process. To avoid races (and data structures corruption) inherent to such situations, data structures must be carefully protected, using semaphores (around sections that might sleep), spin-locks (around non-sleeping sections, to handle SMP machines) etc.