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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