Home Linux Community Forums Programming and Development Kernel Development change user mode value from kernel control path

change user mode value from kernel control path

Link to this post 26 Mar 10


for a special purpose I need to read values in lxrt user mode which are changed in the kernel control path. To avoid expensive system calls I register the address of a user mode variable (int) with a new pointer in the task control path:

typedef struct rt_task_struct {
volatile int *tp_status;

To register the variable I introduced a new function in the api.c file:

RTAI_SYSCALL_MODE int rt_register_v(int variable, void *address, int size)
RT_TASK *rt_current;

if (!access_ok(VERIFY_WRITE, address, size)) {
return -EINVAL;

rt_current = RT_CURRENT;
switch(variable) {
rt_current->tp_status = (int *)address;

In my user space program I use the following code to register the variable:

int anytime_assign_tp_status(const int *tp_status) {
if (tp_status == NULL) {

if (mlock((const void *)tp_status, sizeof(const int )) < 0) {
if (rt_register_v(RT_VAR_TP_STATUS, (void *)tp_status, sizeof(int)) < 0) {
return 0;


I call the function with the following command:

volatile int tp_status = 1;

The registration works fine. In rt_schedule() I am able to access the user mode value with *(rt_current->tp_status). To chance the value from kernel control path I use the following line of code:

if (rt_current->tp_status != NULL) put_user(3, rt_current->tp_status);

This also works fine. But after a lxrt_context_switch(prev, new_task->lnxtsk, cpuid); call in the rt_time_handler function the pointer new_task->tp_status points to the same user mode address but every access to user mode variable like *(new_task->tp_status) freeze the whole system? Furthermore sometimes it is possible to read the value of *(new_task->tp_status) and the content isn’t longer 1 or 3 - its 260348 for example. Does anybody knows the problem? I avoided swapping with the mlock call so why it is not longer possible to access the value after a context switch?

Best regards,


Who we are ?

The Linux Foundation is a non-profit consortium dedicated to the growth of Linux.

More About the foundation...

Frequent Questions

Linux Training / Board