Understanding the Linux Kernel

4.3. Nested Execution of Exception and Interrupt Handlers

A kernel control path consists of the sequence of instructions executed in Kernel Mode to handle an interrupt or an exception. 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.

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 will resume execution as soon as the process gets the processor again.

Since the "Page fault" exception handler never gives rise to further exceptions, at most two kernel control paths associated with exceptions may be stacked, one on top of the other.

In contrast to exceptions, interrupts issued by I/O devices do not refer to data structures specific to the current process, although the kernel control paths that handle them ...