Linux Device Drivers, Second Edition

A Brief Introduction to Race Conditions

Now that you understand how scull’s memory management works, here is a scenario to consider. Two processes, A and B, both have the same scull device open for writing. Both attempt simultaneously to append data to the device. A new quantum is required for this operation to succeed, so each process allocates the required memory and stores a pointer to it in the quantum set.

The result is trouble. Because both processes see the same scull device, each will store its new memory in the same place in the quantum set. If A stores its pointer first, B will overwrite that pointer when it does its store. Thus the memory allocated by A, and the data written therein, will be lost.

This situation is a classic race condition; the results vary depending on who gets there first, and usually something undesirable happens in any case. On uniprocessor Linux systems, the scull code would not have this sort of problem, because processes running kernel code are not preempted. On SMP systems, however, life is more complicated. Processes A and B could easily be running on different processors and could interfere with each other in this manner.

The Linux kernel provides several mechanisms for avoiding and managing race conditions. A full description of these mechanisms will have to wait until Chapter 9, but a beginning discussion is appropriate here.

A semaphore is a general mechanism for controlling access to resources. In its simplest form, a semaphore ...