Re: prepare_to_wait()

["Venky Shankar" <vshankar0@xxxxxxxxx>]

On Tue, Sep 2, 2008 at 9:39 PM, Rik van Riel <riel@xxxxxxxxxxx> wrote:

On Tue, 2 Sep 2008 11:40:59 +0530
"Venky Shankar" <vshankar0@xxxxxxxxx> wrote:

> up(&semaphore);
> ---
> ---
> ---
> prepare_to_wait(queue, &wait, TASK_INTERRUPTIBLE);
> if(condition())
>     schedule();
> finish_wait(queue, &wait);
> ---
> ---
> ---
> down_interruptible(&semaphore);

>  1. The above code can get preempted just after the call to
> prepare_to_wait(), In that case it would not be run again by the scheduler
> since it
>  is in the TASK_INTERRUPTIBLE state. After that when someone wake's it up,
> then the check is made (if condition) and then the normal flow of code.

This is correct.

>  2. If the code does not get preempted after the call to prepare_to_wait(),
> it will still execute the if condition while in TASK_INTERRUPTIBLE state -->
> but a  TASK_INTERRUPTIBLE state is not runnable - i feel this is valid after
> the call to schedule() is made. (i.e. it would not run again after it has
> given up the CPU)

The task continues to run after it has set its state to TASK_INTERRUPTIBLE.
Once it enters schedule() it will be taken off the runqueue and stop
being runnable.

This is ok, because by now it is on a wait queue and eventually it
should get woken up.

 

    >> Thank you very much.

So yes, you understand this code correctly :)

    >> I am happy :)

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