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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,


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