Understanding the Linux Kernel, Second Edition

When Synchronization Is Not Necessary

As already pointed out, the Linux kernel is not preemptive — that is, a running process cannot be preempted (replaced by a higher-priority process) while it remains in Kernel Mode. In particular, the following assertions always hold in Linux:

• No process running in Kernel Mode may be replaced by another process, except when the former voluntarily relinquishes control of the CPU.^[38]

• Interrupt, exception, or softirq handling can interrupt a process running in Kernel Mode; however, when the handler terminates, the kernel control path of the process is resumed.

• A kernel control path performing interrupt handling cannot be interrupted by a kernel control path executing a deferrable function or a system call service routine.

Thanks to these assertions, on uniprocessor systems kernel control paths dealing with nonblocking system calls are atomic with respect to other control paths started by system calls. This simplifies the implementation of many kernel functions: any kernel data structures that are not updated by interrupt, exception, or softirq handlers can be safely accessed. However, if a process in Kernel Mode voluntarily relinquishes the CPU, it must ensure that all data structures are left in a consistent state. Moreover, when it resumes its execution, it must recheck the value of all previously accessed data structures that could be changed. The change could be caused by a different kernel control path, possibly running the same code on behalf ...