Questions about process state manipulation

[Kshemendra KP]

On Tue, Sep 18, 2012 at 9:32 AM, Amit Gupta <gupta.v.amit at gmail.com> wrote:

> Hi All,
>
> I had a question about manipulating process state information in the
> kernel and am curious about the right way to go about it.
> (I'm just messing around with some stuff to try and understand the kernel
> better). I had 2 questions:
>
> 1. I'm trying to create a process in the TASK_INTERRUPTIBLE state in the
> kernel. Since do_fork() creates a process/task that is ready to be
> scheduled, I was thinking what might be a good way to pull it off the
> runqueue/stop its execution after do_fork() returns?
>
>

Assuming you do the change in the do_fork() so that new process moves to
sleep queue
Whatever new process created made to TASK_INTERRUPTIBLE in the do fork, who
will wakeup the new process.
If you just type "ls" commnad also it does not work as ls command calls
fork() + exec().  So your shell fork() and
moves to sleep queue. Similarly lot of fork+exec activities happen in the
system will all move to sleep queue.  Many
deamons running on the system may do fork() or fork + exec() will stop
working.


> 2.I'm curious if someone can explain the correct way to use the
>  "set_task_state()" macro. Its implementation just seems to boil down to an
> assignment to the appropriate member of the task_struct. The question I had
> in mind was, what if the process is already running? Is it safe to do it?
> Don't we need to acquire some sort of lock before we manipulate a
> task_struct ?
>

set_task_state() macro is used for portability reason. Earlier code shows
direct manipulaation fo task_struct->state.
When you execute wait_interruptile() family calls, it is the running
process in kernel  executes set_task_state() .




>
> 3. I would also be interested to read the correct way to use task_lock()
> that locks on alloc_lock field of the task_struct. What exactly is this
> lock for and does acquiring it/or any other lock in the task_struct cause
> the process to be remoted from the runqueue?
>
>
> Would appreciate any pointers to appropriate code/reading or any
> explanation.
> Thanks,
> --
> Amit Gupta
>
>
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