Understanding the Linux Kernel

11.1. Kernel Control Paths

As we said, kernel functions are executed following a request that may be issued in two possible ways:

• A process executing in User Mode causes an exception, for instance by executing an int 0x80 assembly language instruction.

• An external device sends a signal to a Programmable Interrupt Controller by using an IRQ line, and the corresponding interrupt is enabled.

The sequence of instructions executed in Kernel Mode to handle a kernel request is denoted as kernel control path : when a User Mode process issues a system call request, for instance, the first instructions of the corresponding kernel control path are those included in the initial part of the system_call( ) function, while the last instructions are those included in the ret_from_sys_call( ) function.

In Section 4.3 in Chapter 4, a kernel control path was defined as a sequence of instructions executed by the kernel to handle a system call, an exception, or an interrupt. Kernel control paths play a role similar to that of processes, except that they are much more rudimentary: first, no descriptor of any kind is attached to them; second, they are not scheduled through a single function, but rather by inserting sequences of instructions that stop or resume the paths into the kernel code.

In the simplest cases, the CPU executes a kernel control path sequentially from the first instruction to the last. When one of the following events occurs, however, the CPU interleaves kernel control paths:

• A context ...