Day 82 - kthread Internals¶

Date¶

2026-06-22

Objective¶

Learn how Linux kernel threads are created, started, stopped, and managed.

Understand the relationship between:

kthread_create()
wake_up_process()
kthread_run()
kthread_stop()
kthread_should_stop()

Build a userspace simulation of Linux kthread infrastructure and extend it with a simplified kthread_worker framework.

Topics Covered¶

Kernel Thread Fundamentals¶

Studied the purpose of kernel threads and how they differ from workqueues.

Key concepts:

Dedicated execution context
Long-lived background task
Cooperative termination
Driver-owned thread lifecycle

Thread Lifecycle¶

Studied the typical lifecycle of a Linux kernel thread.

kthread_create()
        ↓
CREATED
        ↓
wake_up_process()
        ↓
RUNNING
        ↓
kthread_stop()
        ↓
STOP_REQUESTED
        ↓
thread exits
        ↓
EXITED

A kernel thread is not forcefully terminated.

The thread must periodically check:

kthread_should_stop()

and exit cooperatively.

kthread APIs¶

Studied:

kthread_create()
wake_up_process()
kthread_run()
kthread_should_stop()
kthread_stop()

Relationship:

kthread_run()
    =
kthread_create()
    +
wake_up_process()

TLS-Based Current Task Simulation¶

Implemented a userspace simulation of Linux current.

static __thread struct task_struct *current_task;

Each thread owns an independent TLS copy.

Verified that multiple threads maintain independent task contexts without interfering with each other.

kthread_worker¶

Studied the architecture of kthread_worker and kthread_work.

Architecture:

kthread_work
        ↓
queue_kthread_work()
        ↓
kthread_worker
        ↓
Dedicated kthread

Unlike workqueues, all queued work items execute on the same worker thread.

Typical use cases:

State machines
Ordered command processing
Firmware update workflows
Background driver services

Labs Completed¶

Lab1 - kthread_create() and wake_up_process()¶

Implemented:

kthread_create()
wake_up_process()

Verified:

Thread creation does not start execution
Thread begins execution only after wake_up_process()

Lab2 - kthread_run()¶

Implemented:

kthread_run()

Verified:

kthread_run()
    =
kthread_create()
    +
wake_up_process()

Lab3 - Cooperative Thread Stop¶

Implemented:

kthread_should_stop()
kthread_stop()

Verified:

Stop request propagation
Cooperative thread termination
Thread lifecycle handling

Lab4 - TLS Current Task Simulation¶

Implemented:

static __thread struct task_struct *current_task;

Verified:

Independent task context per thread
Multi-thread isolation
Linux current concept simulation

Lab5 - kthread_worker Fundamentals¶

Implemented:

kthread_worker
kthread_work
queue_kthread_work()

Verified:

Dedicated worker thread execution
Ordered work processing
Single-thread execution model

Example:

WORK-1 tid=135119002138304
WORK-2 tid=135119002138304
WORK-3 tid=135119002138304

All work items executed on the same worker thread.

Lab6 - Worker Synchronization¶

Implemented:

Pending state tracking
Running state tracking
Duplicate queue prevention
Flush synchronization
Worker stop handling

Verified:

Duplicate pending work rejection
Duplicate running work rejection
flush_kthread_worker() waits for active work completion
Worker stop drains pending work before exit

Framework Implemented¶

utils/kthread¶

Implemented:

kthread_create()
wake_up_process()
kthread_run()
kthread_stop()
kthread_should_stop()
kthread_destroy()

Task structure:

struct task_struct
{
    pthread_t thread;

    bool started;
    bool should_stop;

    int (*threadfn)(void *data);
    void *data;

    char name[32];
};

utils/kthread_worker¶

Implemented:

init_kthread_work()
kthread_worker_init()
queue_kthread_work()
flush_kthread_worker()
kthread_worker_stop()

Worker structure:

kthread_worker
    ├── task
    ├── head
    ├── tail
    ├── active_works
    ├── running
    ├── lock
    ├── not_empty
    └── work_done

Key Takeaways¶

Kernel threads provide dedicated execution contexts.
kthread_run() is a convenience wrapper around kthread_create() and wake_up_process().
Thread termination is cooperative through kthread_should_stop().
TLS can be used to simulate Linux current.
kthread_worker executes queued work on a dedicated thread.
Workqueues and kthread workers solve similar problems but provide different execution guarantees.

Next Step¶

Day 83 - Deferred Work Architecture and Selection

Topics:

Deferred execution models
Threaded IRQ
Workqueue
kthread_worker
Dedicated kthread
Driver execution context selection
Deferred work design patterns

Goal:

Understand how Linux drivers choose between threaded IRQs, workqueues, kthread workers, and dedicated kernel threads for deferred processing.