[1 Of course all process switches are performed in Kernel Mode However a process switch may occur only when the current process is going to return in User Mode

• 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 of a separate process.

As you would expect, things are much more complicated in multiprocessor systems. Many CPUs may execute kernel code at the same time, so kernel developers cannot assume that a data structure can be safely accessed just because it is never touched by an interrupt, exception, or softirq handler.

The rest of this chapter describes what to do when synchronization is necessary — i.e., how to prevent data corruption due to unsafe accesses to shared data structures.

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Continue reading here: Synchronization Primitives

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