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  • Book Overview & Buying Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization
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Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

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

By : Kaiwan N. Billimoria
4.5 (6)
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Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

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

4.5 (6)
By: Kaiwan N. Billimoria
Buy this Book

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)
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Preface
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Preface
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Who this book is for
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What this book covers
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To get the most out of this book
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Download the example code files
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Download the color images
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Conventions used
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Get in touch
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Reviews
1
Section 1: Character Device Driver Basics
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Section 1: Character Device Driver Basics
Free Chapter
2
Writing a Simple misc Character Device Driver
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Writing a Simple misc Character Device Driver
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Technical requirements
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Getting started with writing a simple misc character device driver
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Understanding the device basics
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A quick note on the Linux Device Model
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Writing the misc driver code – part 1
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Understanding the connection between the process, the driver, and the kernel
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Handling unsupported methods
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Writing the misc driver code – part 2
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Writing the misc driver code – part 3
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Testing our simple misc driver
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Copying data from kernel to user space and vice versa
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Leveraging kernel APIs to perform the data transfer
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A misc driver with a secret
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Writing the 'secret' misc device driver's code
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Our secret driver – the init code
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Our secret driver – the read method
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Our secret driver – the write method
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Our secret driver – cleanup
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Our secret driver – the user space test app
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Issues and security concerns
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Hacking the secret driver
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Bad driver – buggy read()
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Bad driver – buggy write() – a privesc!
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User space test app modifications
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Device driver modifications
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Let's get root now
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Summary
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Questions
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Further reading
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User-Kernel Communication Pathways
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User-Kernel Communication Pathways
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Technical requirements
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Approaches to communicating/interfacing a kernel driver with a user space C app
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Interfacing via the proc filesystem (procfs)
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Understanding the proc filesystem
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Directories under /proc
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The purpose behind the proc filesystem
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procfs is off-bounds to driver authors
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Using procfs to interface with the user space
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Basic procfs APIs
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The four procfs files we will create
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Trying out the dynamic debug_level procfs control
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Dynamically controlling debug_level via procfs
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A few misc procfs APIs
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Interfacing via the sys filesystem (sysfs)
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Creating a sysfs (pseudo) file in code
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Creating a simple platform device
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Platform devices
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Tying it all together – setting up the device attributes and creating the sysfs file
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The code for implementing our sysfs file and its callbacks
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The "one value per sysfs file" rule
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Interfacing via the debug filesystem (debugfs)
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Checking for the presence of debugfs
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Looking up the debugfs API documentation
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An interfacing example with debugfs
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Creating and using the first debugfs file
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Creating and using the second debugfs file
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Helper debugfs APIs for working on numeric globals
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Removing the debugfs pseudo file(s)
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Seeing a kernel bug – an Oops!
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Debugfs – actual users
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Interfacing via netlink sockets
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Advantages using sockets
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Understanding what a netlink socket is
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Writing the user space netlink socket application
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Writing the kernel-space netlink socket code as a kernel module
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Trying out our netlink interfacing project
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Interfacing via the ioctl system call
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Using ioctl in the user and kernel space
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User space – using the ioctl system call
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Kernel space – using the ioctl system call
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ioctl as a debug interface
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Comparing the interfacing methods – a table
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Summary
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Questions
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Further reading
4
Working with Hardware I/O Memory
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Working with Hardware I/O Memory
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Technical requirements
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Accessing hardware I/O memory from the kernel
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Understanding the issue with direct access
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The solution – mapping via I/O memory or I/O port
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Asking the kernel's permission
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Understanding and using memory-mapped I/O
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Using the ioremap*() APIs
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The newer breed – the devm_* managed APIs
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Obtaining the device resources
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All in one with the devm_ioremap_resource() API
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Looking up the new mapping via /proc/iomem
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MMIO – performing the actual I/O
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Performing 1- to 8-byte reads and writes on MMIO memory regions
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Performing repeating I/O on MMIO memory regions
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Setting and copying on MMIO memory regions
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Understanding and using port-mapped I/O
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PMIO – performing the actual I/O
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A PIO example – the i8042
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Looking up the port(s) via /proc/ioports
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Port I/O – a few remaining points to note
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Summary
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Questions
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Further reading
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Handling Hardware Interrupts
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Handling Hardware Interrupts
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Technical requirements
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Hardware interrupts and how the kernel handles them
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Allocating the hardware IRQ
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Allocating your interrupt handler with request_irq()
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Freeing the IRQ line
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Setting interrupt flags
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Understanding level- and edge-triggered interrupts – a brief note
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Code view 1 – the IXGB network driver
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Implementing the interrupt handler routine
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Interrupt context guidelines – what to do and what not to do
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Don't block – spotting possibly blocking code paths
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Interrupt masking – the defaults and controlling it
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Keep it fast
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Writing the interrupt handler routine itself
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Code view 2 – the i8042 driver's interrupt handler
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Code view 3 – the IXGB network driver's interrupt handler
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IRQ allocation – the modern way – the managed interrupt facility
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Working with the threaded interrupts model
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Employing the threaded interrupt model – the API
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Employing the managed threaded interrupt model – the recommended way
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Code view 4 – the STM32 F7 microcontroller's threaded interrupt handler
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Internally implementing the threaded interrupt
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Why use threaded interrupts?
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Threaded interrupts – to really make it real time
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Constraints when using a threaded handler
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Working with either hardirq or threaded handlers
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Enabling and disabling IRQs
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The NMI
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Viewing all allocated interrupt (IRQ) lines
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Understanding and using top and bottom halves
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Specifying and using a tasklet
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Initializing the tasklet
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Running the tasklet
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Understanding the kernel softirq mechanism
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Available softirqs and what they are for
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Understanding how the kernel runs softirqs
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Running tasklets
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Employing the ksoftirqd kernel threads
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Softirqs and concurrency
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Hardirqs, tasklets, and threaded handlers – what to use when
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Fully figuring out the context
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Viewing the context – examples
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How Linux prioritizes activities
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A few remaining FAQs answered
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Load balancing interrupts and IRQ affinity
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Does the kernel maintain separate IRQ stacks?
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Measuring metrics and latency
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Measuring interrupts with [e]BPF
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Measuring time servicing individual hardirqs
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Measuring time servicing individual softirqs
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Using Ftrace to get a handle on system latencies
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Finding the interrupts disabled worst-case time latency with Ftrace
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Other tools
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Summary
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Questions
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Further reading
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Working with Kernel Timers, Threads, and Workqueues
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Working with Kernel Timers, Threads, and Workqueues
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Technical requirements
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Delaying for a given time in the kernel
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Understanding how to use the *delay() atomic APIs
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Understanding how to use the *sleep() blocking APIs
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Taking timestamps within kernel code
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Let's try it – how long do delays and sleeps really take?
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The "sed" drivers – to demo kernel timers, kthreads, and workqueues
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Setting up and using kernel timers
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Using kernel timers
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Our simple kernel timer module – code view 1
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Our simple kernel timer module – code view 2
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Our simple kernel timer module – running it
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sed1 – implementing timeouts with our demo sed1 driver
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Deliberately missing the bus
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Creating and working with kernel threads
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A simple demo – creating a kernel thread
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Running the kthread_simple kernel thread demo
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The sed2 driver – design and implementation
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sed2 – the design
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sed2 driver – code implementation
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sed2 – trying it out
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Querying and setting the scheduling policy/priority of a kernel thread
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Using kernel workqueues
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The bare minimum workqueue internals
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Using the kernel-global workqueue
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Initializing the kernel-global workqueue for your task – INIT_WORK()
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Having your work task execute – schedule_work()
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Variations of scheduling your work task
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Cleaning up – canceling or flushing your work task
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A quick summary of the workflow
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Our simple work queue kernel module – code view
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Our simple work queue kernel module – running it
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The sed3 mini project – a very brief look
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Summary
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Questions
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Further reading
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Section 2: Delving Deeper
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Section 2: Delving Deeper
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Kernel Synchronization - Part 1
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Kernel Synchronization - Part 1
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Critical sections, exclusive execution, and atomicity
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What is a critical section?
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A classic case – the global i ++
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Concepts – the lock
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A summary of key points
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Concurrency concerns within the Linux kernel
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Multicore SMP systems and data races
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Preemptible kernels, blocking I/O, and data races
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Hardware interrupts and data races
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Locking guidelines and deadlocks
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Mutex or spinlock? Which to use when
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Determining which lock to use – in theory
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Determining which lock to use – in practice
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Using the mutex lock
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Initializing the mutex lock
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Correctly using the mutex lock
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Mutex lock and unlock APIs and their usage
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Mutex lock – via [un]interruptible sleep?
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Mutex locking – an example driver
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The mutex lock – a few remaining points
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Mutex lock API variants
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The mutex trylock variant
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The mutex interruptible and killable variants
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The mutex io variant
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The semaphore and the mutex
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Priority inversion and the RT-mutex
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Internal design
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Using the spinlock
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Spinlock – simple usage
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Spinlock – an example driver
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Test – sleep in an atomic context
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Testing on a 5.4 debug kernel
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Testing on a 5.4 non-debug distro kernel
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Locking and interrupts
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Using spinlocks – a quick summary
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Summary
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Questions
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Further reading
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Kernel Synchronization - Part 2
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Kernel Synchronization - Part 2
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Using the atomic_t and refcount_t interfaces
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The newer refcount_t versus older atomic_t interfaces
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The simpler atomic_t and refcount_t interfaces
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Examples of using refcount_t within the kernel code base
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64-bit atomic integer operators
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Using the RMW atomic operators
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RMW atomic operations – operating on device registers
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Using the RMW bitwise operators
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Using bitwise atomic operators – an example
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Efficiently searching a bitmask
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Using the reader-writer spinlock
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Reader-writer spinlock interfaces
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A word of caution
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The reader-writer semaphore
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Cache effects and false sharing
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Lock-free programming with per-CPU variables
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Per-CPU variables
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Working with per-CPU
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Allocating, initialization, and freeing per-CPU variables
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Performing I/O (reads and writes) on per-CPU variables
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Per-CPU – an example kernel module
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Per-CPU usage within the kernel
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Lock debugging within the kernel
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Configuring a debug kernel for lock debugging
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The lock validator lockdep – catching locking issues early
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Examples – catching deadlock bugs with lockdep
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Example 1 – catching a self deadlock bug with lockdep
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Fixing it
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Example 2 – catching an AB-BA deadlock with lockdep
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lockdep – annotations and issues
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lockdep annotations
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lockdep issues
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Lock statistics
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Viewing lock stats
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Memory barriers – an introduction
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An example of using memory barriers in a device driver
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Summary
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Questions
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Further reading
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Other Books You May Enjoy
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Leave a review - let other readers know what you think

An interfacing example with debugfs

Debugfs, being deliberately designed with a "no particular rules" mindset, makes it the ideal interface to use for debug purposes. Why? It allows you to construct any arbitrary byte stream and send it off to the user space, including a binary "blob" with the debugfs_create_blob() API.

Our previous example kernel modules with procfs and sysfs constructed and used three to four (pseudo) files. For a quick demo with debugfs, we shall just stick to two "files":

  • llkd_dbgfs_show_drvctx: As you'll have no doubt guessed, when read, it will cause the current content of our (by now familiar) "driver context" data structure to be dumped to the console; we shall ensure the pseudo file's mode is read-only (by root).
  • llkd_dbgfs_debug_level: This file's mode shall be read-write (by root only); when read, it will display the current value of debug_level; when an integer is written to it, we shall update...

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