Understanding the Linux Kernel, Second Edition

Nested Execution of Exception and Interrupt Handlers

When handling an interrupt or an exception, the kernel begins a new kernel control path , or separate sequence of instructions. When a process issues a system call request, for instance, the first instructions of the corresponding kernel control path are those that save the content of the registers in the Kernel Mode stack, while the last instructions are those that restore the content of the registers and put the CPU back into User Mode.

Linux design does not allow process switching while the CPU is executing a kernel control path associated with an interrupt. However, such kernel control paths may be arbitrarily nested; an interrupt handler may be interrupted by another interrupt handler, thus giving raise to a nested execution of kernel threads. We emphasize that the current process doesn’t change while the kernel is handling a nested set of kernel control paths.

Assuming that the kernel is bug free, most exceptions can occur only while the CPU is in User Mode. Indeed, they are either caused by programming errors or triggered by debuggers. However, the Page Fault exception may occur in Kernel Mode. This happens when the process attempts to address a page that belongs to its address space but is not currently in RAM. While handling such an exception, the kernel may suspend the current process and replace it with another one until the requested page is available. The kernel control path that handles the Page fault exception ...