Understanding the Linux Kernel, 3rd Edition

Examples of Race Condition Prevention

Kernel developers are expected to identify and solve the synchronization problems raised by interleaving kernel control paths. However, avoiding race conditions is a hard task because it requires a clear understanding of how the various components of the kernel interact. To give a feeling of what’s really inside the kernel code, let’s mention a few typical usages of the synchronization primitives defined in this chapter.

Reference Counters

Reference counters are widely used inside the kernel to avoid race conditions due to the concurrent allocation and releasing of a resource. A reference counter is just an atomic_t counter associated with a specific resource such as a memory page, a module, or a file. The counter is atomically increased when a kernel control path starts using the resource, and it is decreased when a kernel control path finishes using the resource. When the reference counter becomes zero, the resource is not being used, and it can be released if necessary.

The Big Kernel Lock

In earlier Linux kernel versions, a big kernel lock (also known as global kernel lock, or BKL) was widely used. In Linux 2.0, this lock was a relatively crude spin lock, ensuring that only one processor at a time could run in Kernel Mode. The 2.2 and 2.4 kernels were considerably more flexible and no longer relied on a single spin lock; rather, a large number of kernel data structures were protected by many different spin locks. In Linux kernel version 2.6, ...