puting task to TASK_INTERRUPTIBLE before adding it to an wait queue

puting task to TASK_INTERRUPTIBLE before adding it to an wait queue

[Yitzchak Eidus]

the function worker_thread in kernel 2.6.15.6  first put the task to
TASK_INTERRUPTIBLE and only then add itself to an wait queue:
	set_current_state(TASK_INTERRUPTIBLE);
	while (!kthread_should_stop()) {
		add_wait_queue(&cwq->more_work, &wait);
....
my question is, what will happen if the timeslice for the
worker_thread will finished just before it add itself to the wait
queue?
wont it call schedule() that will find the task is in
TASK_INTERRUPTIBLE state and remove it from the runqueue? ( that what
schedule() should do no? )
and then how will the kernel be able to call to worker_thread ever if
it isnt in any list???
thanks for the comments!

Re: puting task to TASK_INTERRUPTIBLE before adding it to an wait queue

[Yitzchak Eidus]

On 3/17/06, Yitzchak Eidus <ieidus@gmail.com> wrote:
> the function worker_thread in kernel 2.6.15.6  first put the task to
> TASK_INTERRUPTIBLE and only then add itself to an wait queue:
>         set_current_state(TASK_INTERRUPTIBLE);
>         while (!kthread_should_stop()) {
>                 add_wait_queue(&cwq->more_work, &wait);
> ....
> my question is, what will happen if the timeslice for the
> worker_thread will finished just before it add itself to the wait
> queue?
> wont it call schedule() that will find the task is in
> TASK_INTERRUPTIBLE state and remove it from the runqueue? ( that what
> schedule() should do no? )
> and then how will the kernel be able to call to worker_thread ever if
> it isnt in any list???
> thanks for the comments!
>

more over the whole loop look like that:
set_current_state(TASK_INTERRUPTIBLE);
	while (!kthread_should_stop()) {
		add_wait_queue(&cwq->more_work, &wait);
		if (list_empty(&cwq->worklist))
			schedule();
		else
			__set_current_state(TASK_RUNNING);
		remove_wait_queue(&cwq->more_work, &wait);

		if (!list_empty(&cwq->worklist))
			run_workqueue(cwq);
		set_current_state(TASK_INTERRUPTIBLE);
	}

what was the logic of putting the
set_current_state(TASK_INTERRUPTIBLE); before the loop and in the last
statement of the loop?

why not use something like this:
while (!kthread_should_stop()) {
		add_wait_queue(&cwq->more_work, &wait);
		set_current_state(TASK_INTERRUPTIBLE);
		if (list_empty(&cwq->worklist))
			schedule();
		else
			__set_current_state(TASK_RUNNING);
		remove_wait_queue(&cwq->more_work, &wait);

		if (!list_empty(&cwq->worklist))
			run_workqueue(cwq);
	}
that do the same thing without putting the task before the loop and in
the loop...?
( unless i am missing something? )

Re: puting task to TASK_INTERRUPTIBLE before adding it to an wait queue

[Mike Galbraith]

On Fri, 2006-03-17 at 00:31 +0200, Yitzchak Eidus wrote:
> the function worker_thread in kernel 2.6.15.6  first put the task to
> TASK_INTERRUPTIBLE and only then add itself to an wait queue:
> 	set_current_state(TASK_INTERRUPTIBLE);
> 	while (!kthread_should_stop()) {
> 		add_wait_queue(&cwq->more_work, &wait);

See dusty old archives...

http://www.ussg.iu.edu/hypermail/linux/kernel/9712.2/0545.html

<quote>
Anyway, the basic race-free wait loop looks like this (there are
variations, but this is one of the basic versions that you find in
various places):


if (should_wait_condition) {
add_wait_queue(..);
repeat:
current->state = TASK_UNINTERRUPTIBLE;
if (should_wait_condition) {
schedule();
goto repeat;
}
remove_wait_queue(..);
current->state = TASK_RUNNING;
}


There are only two important rules:
- you have to add yourself to the wait queue _before_ testing for the
condition.
- you have to mark yourself asleep _before_ testing for the condition.

</quote>

Re: puting task to TASK_INTERRUPTIBLE before adding it to an wait queue

[Andy Lutomirski]

Mike Galbraith wrote:
> On Fri, 2006-03-17 at 00:31 +0200, Yitzchak Eidus wrote:
> 
>>the function worker_thread in kernel 2.6.15.6  first put the task to
>>TASK_INTERRUPTIBLE and only then add itself to an wait queue:
>>	set_current_state(TASK_INTERRUPTIBLE);
>>	while (!kthread_should_stop()) {
>>		add_wait_queue(&cwq->more_work, &wait);
> 
> 
> See dusty old archives...

Also, preempted tasks get rescheduled regardless of state:

http://www.cs.helsinki.fi/linux/linux-kernel/2003-15/0402.html

--Andy