Chapter 1. Introduction¶

This chapter gives an overview of major features of Linux, as well as Unix kernels. This book is based on Linux 2.6.11 source code.

Linux Versus Other Unix-Like Kernels¶

Several differences between Linux and Unix implementations:

• Kernel threading
• Preemptive kernel: Linux 2.6 can arbitrarily interleave execution flows while they are in privileged mode
• Multiprocessor support: Linux 2.6 supports symmetric multiprocessing (SMP)
• STREAMS is not included in Linux

The Process/Kernel Model¶

• Users processes
• Kernel threads:
  1. run in Kernel Mode;
  2. are non-interactive;
  3. created during system startup
• Kernel routines can be activated in:
  1. system call;
  2. exception signaled by a process;
  3. interrupt by a peripheral device;
  4. kernel thread executed

Process Implementation¶

Process descriptor contains registers:

• Program counter (PC) registers
• Stack pointer (SP) registers
• General purpose registers
• Floating point registers
• Processor control registers
• Memory management registers

Reentrant Kernels¶

A kernel control path denotes the sequence of instructions executed by the kernel to handle a system call, an exception, or an interrupt.

Process Address Space¶

Synchronization and Critical Regions¶

Signals and Interprocess Communication¶

• Unix signals
• System V IPC: semaphores, message queues, and shared memory

Process Management¶

• fork(), _exit(), and exec()-like system calls
• wait4()
• Process groups and login sessions

Memory Management¶

• Virtual memory acts as a logical layer between the application memory requests and the hardware Memory Management Unit (MMU).
• Kernel Memory Allocator: Linux’s KMA uses a Slab allocator on top of a buddy system.
• Process virtual address space

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Doubts and Solutions¶

Verbatim¶

Chatper 1 on Linux Versus Other Unix-Like Kernels [p3]:

Linux uses kernel threads in a very limited way to execute a few kernel functions periodically; however, they do not represent the basic execution context abstraction.

Summary¶

Kernel Architecture¶

• The Linux kernel, as with most Unix kernels, is monolithic: each kernel layer is integrated into the whole kernel program and runs in Kernel Mode on behalf of the current process. [p11]