Creating Swait

Topics in this section,

#

Version

Ubuntu

Ubuntu 22.10

Kernel

6.7.9

  • In this program, you are going to learn

  • How to create swaitqueue?

  • What are the different ways to create swaitqueue?

  • How to wakeup the waiting threads once the task is completed?

#include <linux/kthread.h>

bool kthread_should_stop(void);

  • where

    • kthread_should_stop: this is used to determine whether the thread should return now. Whenever the kthread_stop() is called it will be woken and returns true.

    • return type: returns true when the thread is stopped, false when the thread is still in execution

  • Here is an example of how to use the API,

while (!kthread_should_stop())
{
    //add the instructions to be performed during thread execution.
}

  • Here is the function prototype of the API: kthread_run

#include <linux/kthread.h>

#define kthread_run(threadfn, data, namefmt, ...)

  • where

    • kthread_run : is used to create and wake a kthread

    • return type: struct task_struct* (i.e) address of the kthread

    • threadfn: function to executed by kthread

    • data: data pointer for threadfn which can be used to send any possible arguments required for threadfn.

    • namefmt: printf-style format for the thread name which will be displayed on ps output when the thread is in execution.

  • Here is an example of how to use the API,

kthread_run(mythread,NULL,"sample kthread");

#include <linux/kthread.h>

int kthread_stop(struct task_struct *k);

  • where

    • kthread_stop: stops a kthread.

    • return type: returns the return value of threadfn() which is passed as an argument during kthread creation.

    • k: kthread created by one of the API used to created kthread.

  • Here is the example of how to use the API,

kthread_stop(mythread);

# include <linux/swait.h>

#define DECLARE_SWAIT_QUEUE_HEAD(name)                                      \
    struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(name)

  • where

    • DECLARE_SWAIT_QUEUE_HEAD : To declare and initialize swaitqueue head.

    • name : name which is given to the wait queue head on declaration.

  • Here is the example of the API,

DECLARE_SWAIT_QUEUE_HEAD(myswaitqueue);

# include <linux/swait.h>

#define init_swait_queue_head(q)

  • where

    • init_swait_queue_head : initialize swaitqueue head.

    • q : name of the swaitqueue head which is initialized

  • Here is the example of how to use the API,

init_swait_queue_head(myswaitqueue);

# include <linux/swait.h>

# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name)

  • where

  • Here is the example of how to use the API

DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(myswaitqueue);

#include <linux/swait.h>

#define swait_event_exclusive(wq_head, condition)

  • where

    • swait_event_exclusive : sleeps until the condition is true

    • wq_head : the waitqueue to wait on

    • condition : a C expression for the event to wait for

  • Here is an example of how to use the API,

swait_event_exclusive(mywaitqueue,flag != 0);

#include <linux/swait.h>

void swake_up_one(struct swait_queue_head *q);

  • where

    • swake_up_one : wakes up the waitqueue head which is in waiting state.

    • q : waitqueue which is in waiting state.

  • Here is the example of how to use the API

swake_up_one(myswaitqueue);

  • Here is the prototype of module paramter APIs

#include <linux/module.h>

#define MODULE_LICENSE(_license) MODULE_FILE MODULE_INFO(license, _license)
#define MODULE_AUTHOR(_author) MODULE_INFO(author, _author)
#define MODULE_DESCRIPTION(_description) MODULE_INFO(description, _description)

  • where

    • MODULE_LICENSE: tells the kernel what license is used by our module.

    • MODULE_AUTHOR: denotes the author of this kernel module.

    • MODULE_DESCRIPTION: gives a basic idea about what the kernel module does.

    • These information can be found when modinfo command is used which lists out all these above mentioned information.

  • Here is the example of how to use the Module parameter APIs,

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux Usr");
MODULE_DESCRIPTION("Sample kernel module");

  • Here is the prototype of the API: IS_ERR

#include <linux/err.h>

static inline bool __must_check IS_ERR(__force const void *ptr);

  • where

    • IS_ERR: Detects an error pointer

    • return type: return true if it’s an error pointer else return false.

    • ptr: pointer which needs to detected.

  • Here is an example of how to use the API,

if(IS_ERR(sample_ptr)) {
    //instructions to be executed when error ptr is detected
} else {
    //instructions to be executed when error ptr is not detected
}

  • Here is the prototype of the API: pr_info

#include <linux/printk.h>

#define pr_info(fmt, ...) \
    printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)

  • where

    • pr_info: Prints an info-level messages

    • fmt: format string

  • Here is an example of how to use the API,

pr_info("//sample print statement");

#include <linux/sched.h>

extern int wake_up_process(struct task_struct *tsk);

  • where

    • wake_up_process: wake up a specific process

    • return type: returns 1 if the process is woken up, 0 if the process is in running state.

    • tsk: process to be woken up.

  • Here is the example of how to use the API,

wake_up_process(mythread);

  • Here is the example of the API: msleep

#include <linux/delay.h>

void msleep(unsigned int msecs);

  • where

    • msleep: will put it in sleep for a certain amount of msecs time.

    • msecs: time in milliseconds to sleep for

  • Here is the example of how to use the API,

msleep(1000);

  • Here is the prototype of the Driver entry point API’s

#include <linux/module.h>

#define module_init(x)      __initcall(x);
#define module_exit(x)      __exitcall(x);

  • where

    • module_init: driver initialization entry point which will be called at module insertion time.

    • module_exit: driver exit entry point which will be called during the removal of module.

    • x:

      • function to be run at module insertion for module_init function.

      • function to be run when driver is removed for module_exit function.

  • Here is an example of how to use the driver entry point API’s

module_init(myinitmodule);
module_exit(myexitmodule);

  • In this example let’s see how to create swaitqueue and use with kthread.

  • Include the follow header files(.h) to refer the API being used for the execution.

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kthread.h>
#include <linux/swait.h>
#include <linux/delay.h>

  • Add the following module macros to display information about the license, author and description about the module.

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux Usr");
MODULE_DESCRIPTION("Example of DECLARE_SWAIT_QUEUE_HEAD");

  • Declare the thread and waitqueue variables which we are going to create and use in this example

static struct task_struct *wait_thread;
static struct task_struct *wake_thread;
DECLARE_SWAIT_QUEUE_HEAD(wqueue);
int i = 0;

  • Add the module init function which will be executed once we load the kernel module using insmod command.

static int __init kthread_swait_init(void)
{
    pr_info("Inside thread init function\n");
    thread_start();
    return 0;
}

  • Add the thread start function which is called from module init function, creates the thread and starts it’s execution.

void thread_start(void)
{
    wait_thread = kthread_run(wait_thread_fn,NULL,"kthread_swait wait_thread example");
    if (IS_ERR(wait_thread))
        pr_info("error creating thread\n");
    else
        pr_info("kthread created successfully\n");
    wake_up_thread = kthread_run(wake_up_thread_fn,NULL,"kthread_swait wake_thread example");
    if(IS_ERR(wake_up_thread))
        pr_info("error creating thread\n");
    else
        pr_info("kthread created successfully\n");
}

  • Add the module exit function which will be executed once we unload the kernel module using rmmod command.

static void __exit kthread_swait_exit(void)
{
    pr_info("Inside kthread exit function\n");
    thread_stop();
}

  • Add the thread stop function which is called from module exit function, destroys the thread created and stops it’s execution.

void thread_stop(void)
{
    kthread_stop(wake_thread);
    kthread_stop(wait_thread);
    pr_info("destroyed threads\n");
}

  • Add the thread function API which will be called as soon as the kthread is created and is in running state.

static int wait_thread_fn(void *data)
{
    while (!kthread_should_stop())
    {
        swait_event_exclusive(swait_queue,i > 3);
        pr_info("wait thread execution i = %d\n",i);
        msleep(1000);
    }
    return 0;
}

static int wake_thread_fn(void *data)
{
    while(!kthread_should_stop()) {
        pr_info("wake thread execution i = %d\n",i);
        i++;
        swake_up_one(&swait_queue);
        msleep(1000);
    }
    return 0;
}

  • Add the driver entry points which will be executed once the module is inserted or removed from the kernel.

module_init(kthread_swait_init);
module_exit(kthread_swait_exit);

 1#include <linux/init.h>
 2#include <linux/module.h>
 3#include <linux/kernel.h>
 4#include <linux/kthread.h>
 5#include <linux/swait.h>
 6#include <linux/delay.h>
 7
 8MODULE_LICENSE("GPL");
 9MODULE_AUTHOR("linux usr");
10MODULE_DESCRIPTION("Example of declare_swait_queue_head");
11
12static struct task_struct *wait_thread;
13static struct task_struct *wake_thread;
14DECLARE_SWAIT_QUEUE_HEAD(swait_queue);
15int i = 0;
16
17/* wait_thread_fn - executes when the wait_thread is created,
18 * waits for an event completion by wake_thread,
19 * prints a message and sleeps for 1000ms,
20 * stops when kthread_stop is called */
21
22static int wait_thread_fn(void *data)
23{
24    while (!kthread_should_stop())
25    {
26        swait_event_exclusive(swait_queue,i > 3);
27        pr_info("wait thread execution i = %d\n",i);
28        msleep(1000);
29    }
30    return 0;
31}
32
33/* wake_thread_fn - executes when wake_thread is created,
34 * prints a message, increments i value and sleeps for 1000ms,
35 * sends wake up signal to wait_thread to check for condition,
36 * stops when kthread_stop is called */
37
38static int wake_thread_fn(void *data)
39{
40    while(!kthread_should_stop()) {
41        pr_info("wake thread execution i = %d\n",i);
42        i++;
43        swake_up_one(&swait_queue);
44        msleep(1000);
45    }
46    return 1;
47}
48
49/* thread_start - creates thread and starts execution */
50
51void thread_start(void)
52{
53    wait_thread = kthread_run(wait_thread_fn,NULL,"kthread_swait wait_thread example");
54    if (IS_ERR(wait_thread))
55        pr_info("error creating wait_thread\n");
56    else
57        pr_info("successfully created wait thread\n");
58    wake_thread = kthread_run(wake_thread_fn,NULL,"kthread_swait wake_thread example");
59    if (IS_ERR(wake_thread))
60        pr_info("error creating wake thread\n");
61    else
62        pr_info("successfully created wake thread\n");
63}
64
65/* kthread_swait_init - calls thread_start,
66 * executes when the module is loaded */
67
68static int __init kthread_swait_init(void)
69{
70    pr_info("inside kthread init function\n");
71    thread_start();
72    return 0;
73
74}
75
76/* thread_stop - destroys thread and stops execution */
77
78void thread_stop(void)
79{
80    kthread_stop(wake_thread);
81    kthread_stop(wait_thread);
82    pr_info("destroyed threads\n");
83}
84
85/* kthread_swait_exit - calls thread_stop,
86 * executes when the module is loaded */
87
88static void __exit kthread_swait_exit(void)
89{
90    pr_info("inside kthread exit function\n");
91    thread_stop();
92}
93
94module_init(kthread_swait_init);
95module_exit(kthread_swait_exit);

1obj-m += kthread.o
2 
3all:
4	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
5 
6clean:
7	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

  • Run make to compile the kernel source and generate the .ko image.

make -C /lib/modules/6.7.9/build M=$HOME/kthread_examples/
make[1]: Entering directory '/usr/src/linux-headers-6.7.9'
warning: the compiler differs from the one used to build the kernel
The kernel was built by: x86_64-linux-gnu-gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
You are using:           gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
CC [M]  $HOME/kthread_examples/kthread.o
MODPOST $HOME/kthread_examples/Module.symvers
CC [M]  $HOME/kthread_examples/kthread.mod.o
LD [M]  $HOME/kthread_examples/kthread.ko
BTF [M] $HOME/kthread_examples/kthread.ko
Skipping BTF generation for $HOME/kthread_examples/kthread.ko due to unavailability of vmlinux
make[1]: Leaving directory '/usr/src/linux-headers-6.7.9'

  • Check if the .ko is generated or not using ls command.

test@test-V520-15IKL:~$ ls -l
total 360
-rw-rw-r-- 1 test test    713 Mar 18 16:06 kthread.c
-rw-rw-r-- 1 test test 169784 Mar 18 16:08 kthread.ko
-rw-rw-r-- 1 test test     58 Mar 18 16:08 kthread.mod
-rw-rw-r-- 1 test test   1047 Mar 18 16:08 kthread.mod.c
-rw-rw-r-- 1 test test  96512 Mar 18 16:08 kthread.mod.o
-rw-rw-r-- 1 test test  74696 Mar 18 16:08 kthread.o
-rw-rw-r-- 1 test test    161 Mar 18 16:00 Makefile
-rw-rw-r-- 1 test test     58 Mar 18 16:08 modules.order
-rw-rw-r-- 1 test test      0 Mar 18 16:08 Module.symvers

  • Run modinfo command to get the information about the kernel module.

test@test-V520-15IKL:~/.../tc_1$ modinfo kthread.ko
filename:       $HOME/kthread_examples/kthread.ko
description:    Example of declare_swait_queue_head
author:         Linux Usr
license:        GPL
srcversion:     8D2147F67AB01CF0E482DAC
depends:
retpoline:      Y
name:           kthread
vermagic:       6.7.9 SMP preempt mod_unload modversions

  • insert the module using insmod command.

test@test-V520-15IKL:~$ sudo insmod ./kthread.ko

  • check if the module is loaded or not using lsmod command.

test@test-V520-15IKL:~$ sudo lsmod | grep kthread
kthread                16384  0

  • check if the thread is created or not using ps command.

test@test-V520-15IKL:~$ ps -N | grep kthread_swait
6373 ?        00:00:00 kthread_swait wait_thread example
6374 ?        00:00:00 kthread_swait wake_thread example

  • check for the kernel messages from init function and thread function once the module is loaded and thread is created.

test@test-V520-15IKL:~$ sudo dmesg
[  770.083248] inside kthread init function
[  770.083320] successfully created wait thread
[  770.083371] successfully created wake thread
[  770.083377] wake thread execution i = 0
[  771.090390] wake thread execution i = 1
[  772.115023] wake thread execution i = 2
[  773.139696] wake thread execution i = 3
[  773.139723] wait thread execution i = 4
[  774.164227] wait thread execution i = 4
[  774.164244] wake thread execution i = 4
[  775.188857] wait thread execution i = 5

  • remove the module from kernel using rmmod command.

test@test-V520-15IKL:~$ sudo rmmod kthread

  • check if the module is still loaded after removing the kernel module using lsmod if it is not displayed in lsmod output it is verified that the module is removed successfully.

test@test-V520-15IKL:~$ sudo lsmod | grep kthread
test@test-V520-15IKL:~$

  • check if the thread is destroyed using ps command if it is not displayed in ps output we can confirm that the thread is destroyed successfully.

test@test-V520-15IKL:~$ ps -N | grep kthread_swait
test@test-V520-15IKL:~$

  • Check for kernel messages from exit function using dmesg command.

test@test-V520-15IKL:~$ sudo dmesg
[  781.097648] inside kthread exit function
[  781.315947] destroyed threads

  • In this example let’s see how to create swaitqueue head with init_swait_queue_head and use kthread with it.

  • Include the follow header files(.h) to refer the API being used for the execution.

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kthread.h>
#include <linux/swait.h>
#include <linux/delay.h>

  • Add the following module macros to display information about the license, author and description about the module.

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux Usr");
MODULE_DESCRIPTION("Example of init_swait_queue_head");

  • Declare the thread variables which we are going to create and use in this example

static struct task_struct *wait_thread;
static struct task_struct *wake_up_thread;
struct swait_queue_head swait_queue;
int wait_flag = 0;

  • Add the module init function which will be executed once we load the kernel module using insmod command.

static int __init kthread_swait_init(void)
{
    pr_info("Inside kthread init function\n");
    thread_start();
    return 0;
}

  • Add the thread start function called from the module init function which is used to create the thread and execute it.

void thread_start(void)
{
    init_swait_queue_head(&swait_queue);
    pr_info("initialized workqueue head\n");
    wait_thread = kthread_run(wait_thread_fn,NULL,"kthread_swait wait_thread example");
    if (IS_ERR(wait_thread))
        pr_info("error creating thread\n");
    else
        pr_info("kthread created successfully\n");
    wake_thread = kthread_run(wake_up_thread_fn,NULL,"kthread_swait wake_thread example");
    if(IS_ERR(wake_up_thread))
        pr_info("error creating thread\n");
    else
        pr_info("kthread created successfully\n");
}

  • Add the module exit function which will be executed once we unload the kernel module using rmmod command.

static void __exit kthread_swait_exit(void)
{
    pr_info("Inside kthread exit function\n");
    thread_stop();
}

  • Add the thread stop function called from the module exit function which is used to destroy the thread and stop its execution.

void thread_stop(void)
{
    kthread_stop(wake_thread);
    kthread_stop(wait_thread);
    pr_info("destroyed threads\n");
}

  • Add the thread function API which will be called as soon as the kthread is created and is in running state.

static int wait_thread_fn(void *data)
{
    while (!kthread_should_stop())
    {
        swait_event_exclusive(swait_queue,i > 3);
        pr_info("wait thread execution i = %d\n",i);
        msleep(1000);
    }
    return 0;
}

static int wake_thread_fn(void *data)
{
    while(!kthread_should_stop()) {
        pr_info("wake thread execution i = %d\n",i);
        i++;
        swake_up_one(&swait_queue);
        msleep(1000);
    }
    return 1;
}

  • Add the driver entry points which will be executed once the module is inserted or removed from the kernel.

module_init(kthread_swait_init);
module_exit(kthread_swait_exit);

 1#include <linux/init.h>
 2#include <linux/module.h>
 3#include <linux/kernel.h>
 4#include <linux/kthread.h>
 5#include <linux/swait.h>
 6#include <linux/delay.h>
 7
 8MODULE_LICENSE("GPL");
 9MODULE_AUTHOR("linux usr");
10MODULE_DESCRIPTION("Example of init_swait_queue_head");
11
12static struct task_struct *wait_thread;
13static struct task_struct *wake_thread;
14struct swait_queue_head swait_queue;
15int i = 0;
16
17/* wait_thread_fn - executes when the wait_thread is created,
18 * waits for an event completion by wake_thread,
19 * prints a message and sleeps for 1000ms,
20 * stops when kthread_stop is called */
21
22static int wait_thread_fn(void *data)
23{
24    while (!kthread_should_stop())
25    {
26        swait_event_exclusive(swait_queue,i > 3);
27        pr_info("wait thread execution i = %d\n",i);
28        msleep(1000);
29    }
30    return 0;
31}
32
33/* wake_thread_fn - executes when wake_thread is created,
34 * prints a message, increments i value and sleeps for 1000ms,
35 * sends wake up signal to wait_thread to check for condition,
36 * stops when kthread_stop is called */
37
38static int wake_thread_fn(void *data)
39{
40    while(!kthread_should_stop()) {
41        pr_info("wake thread execution i = %d\n",i);
42        i++;
43        swake_up_one(&swait_queue);
44        msleep(1000);
45    }
46    return 1;
47}
48
49/* thread_start - creates thread and starts execution */
50
51void thread_start(void)
52{
53    init_swait_queue_head(&swait_queue);
54    wait_thread = kthread_run(wait_thread_fn,NULL,"kthread_swait wait_thread example");
55    if (IS_ERR(wait_thread))
56        pr_info("error creating wait_thread\n");
57    else
58        pr_info("successfully created wait thread\n");
59    wake_thread = kthread_run(wake_thread_fn,NULL,"kthread_swait wake_thread example");
60    if (IS_ERR(wake_thread))
61        pr_info("error creating wake thread\n");
62    else
63        pr_info("successfully created wake thread\n");
64}
65
66/* kthread_swait_init - calls thread_start,
67 * executes when the module is loaded */
68
69static int __init kthread_swait_init(void)
70{
71    pr_info("inside kthread init function\n");
72    thread_start();
73    return 0;
74
75}
76
77/* thread_stop - destroys thread and stops execution */
78
79void thread_stop(void)
80{
81    kthread_stop(wake_thread);
82    kthread_stop(wait_thread);
83    pr_info("destroyed threads\n");
84}
85
86/* kthread_swait_exit - calls thread_stop,
87 * executes when the module is loaded */
88
89static void __exit kthread_swait_exit(void)
90{
91    pr_info("inside kthread exit function\n");
92    thread_stop();
93}
94
95module_init(kthread_swait_init);
96module_exit(kthread_swait_exit);

1obj-m += kthread.o
2 
3all:
4	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
5 
6clean:
7	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

  • Run make to compile the kernel source and generate the .ko image.

make -C /lib/modules/6.7.9/build M=$HOME/kthread_examples/
make[1]: Entering directory '/usr/src/linux-headers-6.7.9'
warning: the compiler differs from the one used to build the kernel
The kernel was built by: x86_64-linux-gnu-gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
You are using:           gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
CC [M]  $HOME/kthread_examples/kthread.o
MODPOST $HOME/kthread_examples/Module.symvers
CC [M]  $HOME/kthread_examples/kthread.mod.o
LD [M]  $HOME/kthread_examples/kthread.ko
BTF [M] $HOME/kthread_examples/kthread.ko
Skipping BTF generation for $HOME/kthread_examples/kthread.ko due to unavailability of vmlinux
make[1]: Leaving directory '/usr/src/linux-headers-6.7.9'

  • Check if the .ko is generated or not using ls command.

test@test-V520-15IKL:~$ ls -l
total 360
-rw-rw-r-- 1 test test    713 Mar 18 16:06 kthread.c
-rw-rw-r-- 1 test test 169784 Mar 18 16:08 kthread.ko
-rw-rw-r-- 1 test test     58 Mar 18 16:08 kthread.mod
-rw-rw-r-- 1 test test   1047 Mar 18 16:08 kthread.mod.c
-rw-rw-r-- 1 test test  96512 Mar 18 16:08 kthread.mod.o
-rw-rw-r-- 1 test test  74696 Mar 18 16:08 kthread.o
-rw-rw-r-- 1 test test    161 Mar 18 16:00 Makefile
-rw-rw-r-- 1 test test     58 Mar 18 16:08 modules.order
-rw-rw-r-- 1 test test      0 Mar 18 16:08 Module.symvers

  • Run modinfo command to get the information about the kernel module.

test@test-V520-15IKL:~/.../tc_1$ modinfo kthread.ko
filename:       $HOME/kthread_examples/kthread.ko
description:    Example of init_swait_queue_head
author:         Linux Usr
license:        GPL
srcversion:     8D2147F67AB01CF0E482DAC
depends:
retpoline:      Y
name:           kthread
vermagic:       6.7.9 SMP preempt mod_unload modversions

  • insert the module using insmod command.

test@test-V520-15IKL:~$ sudo insmod ./kthread.ko

  • check if the module is loaded or not using lsmod command.

test@test-V520-15IKL:~$ sudo lsmod | grep kthread
kthread                16384  0

  • check if the thread is created or not using ps command.

test@test-V520-15IKL:~$ ps -N | grep kthread_swait
7978 ?        00:00:00 kthread_swait wait_thread example
7979 ?        00:00:00 kthread_swait wake_thread example

  • check for the kernel messages from init function and thread function once the module is loaded and thread is created.

test@test-V520-15IKL:~$ sudo dmesg
[ 2142.952854] inside kthread init function
[ 2142.952990] successfully created wait thread
[ 2142.953054] successfully created wake thread
[ 2142.953060] wake thread execution i = 0
[ 2143.979965] wake thread execution i = 1
[ 2145.003735] wake thread execution i = 2
[ 2146.027725] wake thread execution i = 3
[ 2146.027844] wait thread execution i = 4
[ 2147.051539] wake thread execution i = 4
[ 2147.051553] wait thread execution i = 5

  • remove the module from kernel using rmmod command.

test@test-V520-15IKL:~$ sudo rmmod kthread

  • check if the module is still loaded after removing the kernel module using lsmod if it is not displayed in lsmod output it is verified that the module is removed successfully.

test@test-V520-15IKL:~$ sudo lsmod | grep kthread
test@test-V520-15IKL:~$

  • check if the thread is destroyed using ps command if it is not displayed in ps output we can confirm that the thread is destroyed successfully.

test@test-V520-15IKL:~$ ps -N | grep kthread_swait
test@test-V520-15IKL:~$

  • Check for kernel messages from exit function using dmesg command.

test@test-V520-15IKL:~$ sudo dmesg
[ 2156.374391] inside kthread exit function
[ 2158.314468] destroyed threads

  • In this example let’s see how to create swaitqueue using DECLARE_SWAIT_QUEUE_HEAD_ONSTACK and use kthread with it.

  • Include the follow header files(.h) to refer the API being used for the execution.

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kthread.h>
#include <linux/swait.h>
#include <linux/delay.h>

  • Add the following module macros to display information about the license, author and description about the module.

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux Usr");
MODULE_DESCRIPTION("example of declare_swait_queue_head_onstack");

  • Declare the thread and completion variables which we are going to create and use in this example

static struct task_struct *wait_thread;
static struct task_struct *wake_thread;
DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(swait_queue);
int i = 0;

  • Add the module init function which will be executed once we load the kernel module using insmod command.

static int __init kthread_swait_init(void)
{
    pr_info("Inside kthread init function\n");
    thread_start();
    return 0;
}

  • Add the thread start function called from module init function, which creates the thread, wakes it up and starts its execution.

void thread_start(void)
{
    wait_thread = kthread_run(wait_thread_fn,NULL,"kthread_swait wait_thread example");
    if (IS_ERR(wait_thread))
        pr_info("error creating thread\n");
    else
        pr_info("kthread created successfully\n");
    wake_up_thread = kthread_run(wake_up_thread_fn,NULL,"kthread_swait wake_thread example");
    if(IS_ERR(wake_up_thread))
        pr_info("error creating thread\n");
    else
        pr_info("kthread created successfully\n");
}

  • Add the module exit function which will be executed once we unload the kernel module using rmmod command.

static void __exit kthread_swait_exit(void)
{
    pr_info("Inside kthread exit function\n");
    thread_stop();
}

  • Add the thread stop function which is called from module exit function, which destroys the thread and stops the execution.

void thread_stop(void)
{
    kthread_stop(wake_thread);
    kthread_stop(wait_thread);
    pr_info("destroyed threads\n");
}

  • Add the thread function API which will be called as soon as the kthread is created and is in running state.

static int wait_thread_fn(void *data)
{
    while (!kthread_should_stop())
    {
        swait_event_exclusive(swait_queue,i > 3);
        pr_info("wait thread execution i = %d\n",i);
        msleep(1000);
    }
    return 0;
}

static int wake_thread_fn(void *data)
{
    while(!kthread_should_stop()) {
        pr_info("wake thread execution i = %d\n",i);
        i++;
        swake_up_one(&swait_queue);
        msleep(1000);
    }
    return 1;
}

  • Add the driver entry points which will be executed once the module is inserted or removed from the kernel.

module_init(kthread_swait_init);
module_exit(kthread_swait_exit);

 1#include <linux/init.h>
 2#include <linux/module.h>
 3#include <linux/kernel.h>
 4#include <linux/kthread.h>
 5#include <linux/swait.h>
 6#include <linux/delay.h>
 7
 8MODULE_LICENSE("GPL");
 9MODULE_AUTHOR("linux usr");
10MODULE_DESCRIPTION("Example of declare_swait_queue_head_onstack");
11
12static struct task_struct *wait_thread;
13static struct task_struct *wake_thread;
14DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(swait_queue);
15int i = 0;
16
17/* wait_thread_fn - executes when the wait_thread is created,
18 * waits for an event completion by wake_thread,
19 * prints a message and sleeps for 1000ms,
20 * stops when kthread_stop is called */
21
22static int wait_thread_fn(void *data)
23{
24    while (!kthread_should_stop())
25    {
26        swait_event_exclusive(swait_queue,i > 3);
27        pr_info("wait thread execution i = %d\n",i);
28        msleep(1000);
29    }
30    return 0;
31}
32
33/* wake_thread_fn - executes when wake_thread is created,
34 * prints a message, increments i value and sleeps for 1000ms,
35 * sends wake up signal to wait_thread to check for condition,
36 * stops when kthread_stop is called */
37
38static int wake_thread_fn(void *data)
39{
40    while(!kthread_should_stop()) {
41        pr_info("wake thread execution i = %d\n",i);
42        i++;
43        swake_up_one(&swait_queue);
44        msleep(1000);
45    }
46    return 1;
47}
48
49/* thread_start - creates thread and starts execution */
50
51void thread_start(void)
52{
53    wait_thread = kthread_run(wait_thread_fn,NULL,"kthread_swait wait_thread example");
54    if (IS_ERR(wait_thread))
55        pr_info("error creating wait_thread\n");
56    else
57        pr_info("successfully created wait thread\n");
58    wake_thread = kthread_run(wake_thread_fn,NULL,"kthread_swait wake_thread example");
59    if (IS_ERR(wake_thread))
60        pr_info("error creating wake thread\n");
61    else
62        pr_info("successfully created wake thread\n");
63}
64
65/* kthread_swait_init - calls thread_start,
66 * executes when the module is loaded */
67
68static int __init kthread_swait_init(void)
69{
70    pr_info("inside kthread init function\n");
71    thread_start();
72    return 0;
73
74}
75
76/* thread_stop - destroys thread and stops execution */
77
78void thread_stop(void)
79{
80    kthread_stop(wake_thread);
81    kthread_stop(wait_thread);
82    pr_info("destroyed threads\n");
83}
84
85/* kthread_swait_exit - calls thread_stop,
86 * executes when the module is loaded */
87
88static void __exit kthread_swait_exit(void)
89{
90    pr_info("inside kthread exit function\n");
91    thread_stop();
92}
93
94module_init(kthread_swait_init);
95module_exit(kthread_swait_exit);

1obj-m += kthread.o
2 
3all:
4	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
5 
6clean:
7	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

  • Run make to compile the kernel source and generate the .ko image.

make -C /lib/modules/6.7.9/build M=$HOME/kthread_examples/
make[1]: Entering directory '/usr/src/linux-headers-6.7.9'
warning: the compiler differs from the one used to build the kernel
The kernel was built by: x86_64-linux-gnu-gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
You are using:           gcc (Ubuntu 12.2.0-3ubuntu1) 12.2.0
CC [M]  $HOME/kthread_examples/kthread.o
MODPOST $HOME/kthread_examples/Module.symvers
CC [M]  $HOME/kthread_examples/kthread.mod.o
LD [M]  $HOME/kthread_examples/kthread.ko
BTF [M] $HOME/kthread_examples/kthread.ko
Skipping BTF generation for $HOME/kthread_examples/kthread.ko due to unavailability of vmlinux
make[1]: Leaving directory '/usr/src/linux-headers-6.7.9'

  • Check if the .ko is generated or not using ls command.

test@test-V520-15IKL:~$ ls -l
total 360
-rw-rw-r-- 1 test test    713 Mar 18 16:06 kthread.c
-rw-rw-r-- 1 test test 169784 Mar 18 16:08 kthread.ko
-rw-rw-r-- 1 test test     58 Mar 18 16:08 kthread.mod
-rw-rw-r-- 1 test test   1047 Mar 18 16:08 kthread.mod.c
-rw-rw-r-- 1 test test  96512 Mar 18 16:08 kthread.mod.o
-rw-rw-r-- 1 test test  74696 Mar 18 16:08 kthread.o
-rw-rw-r-- 1 test test    161 Mar 18 16:00 Makefile
-rw-rw-r-- 1 test test     58 Mar 18 16:08 modules.order
-rw-rw-r-- 1 test test      0 Mar 18 16:08 Module.symvers

  • Run modinfo command to get the information about the kernel module.

test@test-V520-15IKL:~/.../tc_1$ modinfo kthread.ko
filename:       $HOME/kthread_examples/kthread.ko
description:    Example of declare_swait_queue_head_onstack
author:         Linux Usr
license:        GPL
srcversion:     8D2147F67AB01CF0E482DAC
depends:
retpoline:      Y
name:           kthread
vermagic:       6.7.9 SMP preempt mod_unload modversions

  • insert the module using insmod command.

test@test-V520-15IKL:~$ sudo insmod ./kthread.ko

  • check if the module is loaded or not using lsmod command.

test@test-V520-15IKL:~$ sudo lsmod | grep kthread
kthread                16384  0

  • check if the thread is created or not using ps command.

test@test-V520-15IKL:~$ ps -N | grep kthread_swait
8518 ?        00:00:00 kthread_swait wait_thread example
8519 ?        00:00:00 kthread_swait wake_thread example

  • check for the kernel messages from init function and thread function once the module is loaded and thread is created.

test@test-V520-15IKL:~$ sudo dmesg
[ 2969.952183] inside kthread init function
[ 2969.952345] successfully created wait thread
[ 2969.952482] successfully created wake thread
[ 2969.952540] wake thread execution i = 0
[ 2970.986614] wake thread execution i = 1
[ 2972.010535] wake thread execution i = 2
[ 2973.034516] wake thread execution i = 3
[ 2973.034627] wait thread execution i = 4
[ 2974.058501] wake thread execution i = 4
[ 2974.058504] wait thread execution i = 4
[ 2975.082471] wake thread execution i = 5
[ 2975.082471] wait thread execution i = 5

  • remove the module from kernel using rmmod command.

test@test-V520-15IKL:~$ sudo rmmod kthread

  • check if the module is still loaded after removing the kernel module using lsmod if it is not displayed in lsmod output it is verified that the module is removed successfully.

test@test-V520-15IKL:~$ sudo lsmod | grep kthread
test@test-V520-15IKL:~$

  • check if the thread is destroyed using ps command if it is not displayed in ps output we can confirm that the thread is destroyed successfully.

test@test-V520-15IKL:~$ ps -N | grep kthread_swait
test@test-V520-15IKL:~$

  • Check for kernel messages from exit function using dmesg command.

test@test-V520-15IKL:~$ sudo dmesg
[ 2978.622086] inside kthread exit function
[ 2980.202260] destroyed threads

kthread API

Learning

kthread_run

Create and wake a thread

kthread_should_stop

To determine when thread should exit

kthread_stop

Stop a thread created by kthread_create

Workqueue API

Learning

DECLARE_SWAIT_QUEUE_HEAD

Declare swaitqueue head

init_swait_queue_head

Initialize swaitqueue head

DECLARE_SWAIT_QUEUEU_HEAD_ONSTACK

Declare swaitqueue head on current stack

swait_event_exclusive

sleep until a condition gets true

swake_up_one

wakes up the waiting queue

API

Learning

MODULE_LICENSE

Used to denote the license used in the kernel module

MODULE_AUTHOR

Used to mention the author of the kernel module

MODULE_DESCRIPTION

Used to describe what the module does

IS_ERR

Detects an error pointer

wake_up_process

wake up a specific process

msleep

will put in sleep for a certain amount of msecs time.

module_init

Driver initialization entry point

module_exit

Driver exit entry point

pr_info

Print an info-level message