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

Fixing it

Now that we understand the issue here, how do we fix it? Seeing lockdep's report (Figure 7.9) and interpreting it, it's quite simple: (as mentioned) since the task structure spinlock named alloc_lock is already taken at the start of the do-while loop (via task_lock(t)), ensure that before calling the get_task_comm() routine (which internally takes and releases this same lock), you unlock it, then perform get_task_comm(), then lock it again.

The following screenshot (Figure 7.10) shows the difference (via the diff(1) utility) between the older buggy version (ch13/3_lockdep/buggy_thrdshow_eg/thrd_showall_buggy.c) and the newer fixed version of our code (ch13/3_lockdep/fixed_lockdep/thrd_showall_fixed.c):

Figure 7.10 – (Partial) screenshot showing the key part of the difference between the buggy and fixed versions of our demo thrdshow LKM

Great; another example follows – that of catching an AB-BA deadlock!