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)
1
Section 1: Character Device Driver Basics
3
User-Kernel Communication Pathways
5
Handling Hardware Interrupts
6
Working with Kernel Timers, Threads, and Workqueues
7
Section 2: Delving Deeper

Running tasklets

A word on the internals of tasklet invocation: we understand that the tasklet softirq runs via tasklet_schedule(). This API ends up invoking the kernel's internal __tasklet_schedule_common() function (https://elixir.bootlin.com/linux/v5.4/source/kernel/softirq.c#L471), which internally calls raise_softirq_irqoff(softirq_nr) (https://elixir.bootlin.com/linux/v5.4/source/kernel/softirq.c#L423). This raises the softirq_nr softirq; for a regular tasklet, this value is TASKLET_SOFTIRQ, whereas when the tasklet is scheduled via the tasklet_hi_schedule() API, is value is HI_SOFTIRQ, the highest priority softirq! Use it rarely, if ever.

We now know that the "schedule" functionality has set up the softirq; here, the actual execution takes place when the softirqs at that priority level (0 or 6 here) actually run. The function that runs softirqs is called do_softirq(); for the regular tasklet, it ends up calling...