Understanding the Linux Kernel

4.1. The Role of Interrupt Signals

As the name suggests, interrupt signals provide a way to divert the processor to code outside the normal flow of control. When an interrupt signal arrives, the CPU must stop what it's currently doing and switch to a new activity; it does this by saving the current value of the program counter (i.e., the content of the eip and cs registers) in the Kernel Mode stack and by placing an address related to the interrupt type into the program counter.

There are some things in this chapter that will remind you of the context switch we described in the previous chapter, carried out when a kernel substitutes one process for another. But there is a key difference between interrupt handling and process switching: the code executed by an interrupt or by an exception handler is not a process. Rather, it is a kernel control path that runs on behalf of the same process that was running when the interrupt occurred (see Section 4.3). As a kernel control path, the interrupt handler is lighter than a process (it has less context and requires less time to set up or tear down).

Interrupt handling is one of the most sensitive tasks performed by the kernel, since it must satisfy the following constraints:

• Interrupts can come at any time, when the kernel may want to finish something else it was trying to do. The kernel's goal is therefore to get the interrupt out of the way as soon as possible and defer as much processing as it can. For instance, suppose a block of data ...