Hi All,

While task switch from CPU 0 to another CPU 1, user stack dump (using __get_user) giving Error while when again return back to CPU 0 it giving correct value.

I have noticed "task->mm->start_stack" never change in both cases.

Below given some result for reference:
================================================================
-------------- CPU 0 -------------------
task stack info : pid(518) stack area (0xbe8b8000 ~ 0xbe8cd000)
pid(518) stack vma (0xbe8b8000 ~ 0xbe8cd000)
User Stack: (0xbe8ccc3c to 0xbe8ccda0)
cc20: be8ccc4c
cc40: be8ccda4 00000001 00000000 4006f4c4 40197000 be8ccda4 00000001 00008474
cc60: 000000e8 00000000 00000000 40197000 00000000 00000000 000000e8 00000000
cc80: be8ccc50 4006f47c 00000000 00000000 00000000 00000000 00000000 00000000
cca0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000

-------------- CPU 1 -------------------
pid(518) stack vma (0xbe8b8000 ~ 0xbe8cd000)
User Stack: (0xbe8ccc3c to 0xbe8ccda0)
cc20: 00000000
cc40: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
cc60: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
cc80: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
================================================================

How do you call __get_user? Is it in user space?

no I am calling in Kernel space

The issue has been resolved on activating the task memory on the CPU context. but i am little bit worried that its caused bad effect on the system.

If the issue had been resolved, the __get_user is called in user context. Otherwise you should get trouble. I am caution of activating the task memory on the CPU context. Do you mean that the page isn't active or isn't loaded into memory at the moment when the task is switched to CPU1?

Yeah you are right. actually for debugging i want to see the dump user stack of the task through the kernel thread. but problem was that when my kernel thread and user task are running in different CPU then stack dump(using __get_user) giving Error. the error get resolved when i have task activated memory before dumping the stack( using activate_mm call before __get_user).
Now my questions are--
what are the bad effect on the system due to activating task memory.
are other running tasks also affected due to this ??? if yes how to protect ??

You can try following case and see what happen.
Your kernel thread and user task are running at same time and in different CPU.