Understanding the Linux Kernel

11.4. The Linux/SMP Kernel

Linux 2.2 support for SMP is compliant with Version 1.4 of the Intel MultiProcessor Specification, which establishes a multiprocessor platform interface standard while maintaining full PC/AT binary compatibility.

As we have seen in Section 11.2.1 earlier in this chapter, race conditions are relatively limited in Linux on a uniprocessor system, so interrupt disabling and kernel semaphores can be used to protect data structures that are asynchronously accessed by interrupt or exception handlers. In a multiprocessor system, however, things are much more complicated: several processes may be running in Kernel Mode, and therefore data structure corruption can occur even if no running process is preempted. The usual way to synchronize access to SMP kernel data structures is by means of semaphores and spin locks (see Section 11.4.2).

Before discussing in detail how Linux 2.2 serializes the accesses to kernel data structures in multiprocessor systems, let us make a brief digression to how this goal was achieved when Linux first introduced SMP support. In order to facilitate the transition from a uniprocessor kernel to a multiprocessor one, the old 2.0 version of Linux/SMP adopted this drastic rule:

At any given instant, at most one processor is allowed to access the kernel data structures and to handle the interrupts.

This rule dictates that each processor wishing to access the kernel data structures must get a global lock. As long as it holds the lock, it ...