Re: Changing the state of a process

[Rajaram Suryanarayanan <rajaram_linux@xxxxxxxxx>]

Jan Hudec <bulb@xxxxxx> wrote:

On Thu, Dec 22, 2005 at 22:41:48 -0800, Rajaram Suryanarayanan wrote:
> Hi,
>
> I wrote a simple kernel module to play with the process descriptor structure of a process. I have given the code below.
>
> int init_module(void)
> {
>
> printk(KERN_INFO "Hello Process \n");
> printk(KERN_INFO "Parent process ID :%d \n",current->parent->pid);
> printk(KERN_INFO "Parent process state :%d \n",current->parent->state);
> printk(KERN_INFO "Parent process uid :%d \n",current->parent->uid);
> printk(KERN_INFO "Parent process priority :%d \n",current->parent->prio);
>
> /* Change the parent process state to unrannable */
> current->parent->state= -1;
> return 0;
>
> }
>
> By this code, I am trying to print the details of the parent process of
> the current process ( current process is insmod and its parent process is
> the bash shell from which I issued insmod command, if I am not wrong ).

Yes, you are right.

> Also I try to change the state of the bash shell to -1 ( unrunnable ).

I see it in the comment, but that state does not seem to actually exist.

> I see that the current->parent->state is printed as 1 . I see that process
> state > 0 is stopped , as defined in "struct task_struct". Then why it is
> printed as 1 when the bash shell is actually running ?

No, it's not actually running. It's waiting for it's child to exit or be
stopped, inside wait syscall. State 1 is TASK_INTERRUPTIBLE.

> Also by changing the state of the bash shell to -1, I expected it to hang
> or something like that. But that did not happen.
>
> Could anyone please let me know what is wrong with my approach..?

Don't use the numbers. Use the symbolic values TASK_RUNNING,
TASK_INTERRUPTIBLE, TASK_UNINTERRUPTIBLE and so on.

However, that's not the major problem. The major problem is, that it's only
permitted to alter your own state (that is set_current_state) except
well-defined case of waking up a process on a wait-queue.

--
Jan 'Bulb' Hudec

Thanks Jan for the informative reply.

Actually, I saw the definition of "struct task_struct" in the file include/linux/sched.h for the different states of the process. There it was -1 for unrunnable etc..

 

struct task_struct {
         volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
         struct thread_info *thread_info;
         atomic_t usage;
         unsigned long flags;    /* per process flags, defined below */
         unsigned long ptrace;

 

Can you please throw some light on how the kernel reacts when I try to change the state of the parent process, ie not my own state..? Because I do not see any error thrown up. How does kernel detect this ?

Also I understand that bash shell is in TASK_INTERRUPTIBLE state, because it is waiting for insmod to complete. If possible, pls tell me how it is waken up by SIGCHLD signal when insmod completes.

 

Thanks,

Rajaram. 

 

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