Book Image

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

By : Kaiwan N. Billimoria
Book Image

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

By: Kaiwan N. Billimoria

Overview of this book

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization is an ideal companion guide to the Linux Kernel Programming book. This book provides a comprehensive introduction for those new to Linux device driver development and will have you up and running with writing misc class character device driver code (on the 5.4 LTS Linux kernel) in next to no time. You'll begin by learning how to write a simple and complete misc class character driver before interfacing your driver with user-mode processes via procfs, sysfs, debugfs, netlink sockets, and ioctl. You'll then find out how to work with hardware I/O memory. The book covers working with hardware interrupts in depth and helps you understand interrupt request (IRQ) allocation, threaded IRQ handlers, tasklets, and softirqs. You'll also explore the practical usage of useful kernel mechanisms, setting up delays, timers, kernel threads, and workqueues. Finally, you'll discover how to deal with the complexity of kernel synchronization with locking technologies (mutexes, spinlocks, and atomic/refcount operators), including more advanced topics such as cache effects, a primer on lock-free techniques, deadlock avoidance (with lockdep), and kernel lock debugging techniques. By the end of this Linux kernel book, you'll have learned the fundamentals of writing Linux character device driver code for real-world projects and products.
Table of Contents (11 chapters)
Section 1: Character Device Driver Basics
User-Kernel Communication Pathways
Handling Hardware Interrupts
Working with Kernel Timers, Threads, and Workqueues
Section 2: Delving Deeper

Deliberately missing the bus

The part I left out earlier is an interesting wrinkle: just before the second timestamp is taken, we insert a bit of code to deliberately miss the sacrosanct deadline! How? It's really very simple:

static void encrypt_decrypt_payload(int work, struct sed_ds *kd, struct sed_ds *kdret)
[ ... ]
// work done!
if (make_it_fail == 1)
msleep(TIMER_EXPIRE_MS + 1);
t2 = ktime_get_real_ns();

make_it_fail is a module parameter that is set to 0 by default; thus, only if you want to live dangerously (yes, a bit exaggerated!) should you pass it as 1. Let's try it out and see our kernel timer expire. The user mode app will detect this and report the failure as well:

Figure 5.4 – Our sed1 mini-project running with the make_it_fail module parameter set to 1, causing the deadline to be missed

This time, the deadline is exceeded before the timer is canceled, thus causing it to expire and...