Re: questions on wait_event ...

[Steven Rostedt <rostedt@goodmis.org>]

Please include CCs of people who respond to you or you might not ever
get a response.  There's too much traffic on the LKML, your email may
get lost in the noise.

On Fri, 2005-12-23 at 07:46 +0600, Alexey Shinkin wrote:
> Look:
> 
> We call wait_event() , condition is FALSE at the moment  :
> 
>     do {
>          if (condition)
>                  break;
>     /* and here we have condition changed  to TRUE  */
>     /*  process is NOT in any wait queue yet  */
>     /*  then  unroll     __wait_event(wq, condition);        */
> 
>      do {							DEFINE_WAIT(__wait);					for (;;) {
> 	prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE);
> 
>           /* at this point condition is TRUE , process is in a wait queue 
> and its state is
>              TASK_UNINTERRUPTIBLE.   If  rescheduling happens now the 
> process will asleep,
>               despite of condition is  TRUE . And will not be woken up until 
> next wake_up happens
>               Is that correct ?  */

OHHH! Your question is about __preemption__!!!

That's a completely different story, because if a process gets
preempted, it will _not_ be taken off the runqueue even if it's state is
in TASK_UNINTERRUPTIBLE.  Otherwise, there would be lots of places in
the kernel that is broken.

from schedule in sched.c:

/* when preempted, the preempt_count gets "PREEMPT_ACTIVE"
   so the following if will not be entered */

	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
		switch_count = &prev->nvcsw;
		if (unlikely((prev->state & TASK_INTERRUPTIBLE) &&
				unlikely(signal_pending(prev))))
			prev->state = TASK_RUNNING;
		else {
			if (prev->state == TASK_UNINTERRUPTIBLE)
				rq->nr_uninterruptible++;

/* Here we would have taken off the task from the runqueue
   but we don't, so the task _will_ wake up again when it is
   scheduled back in. */

			deactivate_task(prev, rq);
		}
	}

-- Steve

> 	if (condition)						     break;
>                     schedule();
>         }
>    finish_wait(&wq, &__wait);
> } while (0)
>    /*  end of unroll __wait_event*/
> 
> } while (0)
> 
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