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SubjectRe: [PATCH for 2.5] preemptible kernel
Nigel Gamble wrote:
>
> On Tue, 20 Mar 2001, Roger Larsson wrote:
> > One little readability thing I found.
> > The prev->state TASK_ value is mostly used as a plain value
> > but the new TASK_PREEMPTED is or:ed together with whatever was there.
> > Later when we switch to check the state it is checked against TASK_PREEMPTED
> > only. Since TASK_RUNNING is 0 it works OK but...
>
> Yes, you're right. I had forgotten that TASK_RUNNING is 0 and I think I
> was assuming that there could be (rare) cases where a task was preempted
> while prev->state was in transition such that no other flags were set.
> This is, of course, impossible given that TASK_RUNNING is 0. So your
> change makes the common case more obvious (to me, at least!)
>
> > --- sched.c.nigel Tue Mar 20 18:52:43 2001
> > +++ sched.c.roger Tue Mar 20 19:03:28 2001
> > @@ -553,7 +553,7 @@
> > #endif
> > del_from_runqueue(prev);
> > #ifdef CONFIG_PREEMPT
> > - case TASK_PREEMPTED:
> > + case TASK_RUNNING | TASK_PREEMPTED:
> > #endif
> > case TASK_RUNNING:
> > }
> >
> >
> > We could add all/(other common) combinations as cases
> >
> > switch (prev->state) {
> > case TASK_INTERRUPTIBLE:
> > if (signal_pending(prev)) {
> > prev->state = TASK_RUNNING;
> > break;
> > }
> > default:
> > #ifdef CONFIG_PREEMPT
> > if (prev->state & TASK_PREEMPTED)
> > break;
> > #endif
> > del_from_runqueue(prev);
> > #ifdef CONFIG_PREEMPT
> > case TASK_RUNNING | TASK_PREEMPTED:
> > case TASK_INTERRUPTIBLE | TASK_PREEMPTED:
> > case TASK_UNINTERRUPTIBLE | TASK_PREEMPTED:
> > #endif
> > case TASK_RUNNING:
> > }
> >
> >
> > Then the break in default case could almost be replaced with a BUG()...
> > (I have not checked the generated code)
>
> The other cases are not very common, as they only happen if a task is
> preempted during the short time that it is running while in the process
> of changing state while going to sleep or waking up, so the default case
> is probably OK for them; and I'd be happier to leave the default case
> for reliability reasons anyway.

Especially since he forgot:

TASK_ZOMBIE
TASK_STOPPED
TASK_SWAPPING

I don't know about the last two but TASK_ZOMBIE must be handled
correctly or the task will never clear.

In general, a task must run till it gets to schedule() before the actual
state is "real" so the need for the TASK_PREEMPT.

The actual code generated with what you propose should be the same (even
if TASK_RUNNING != 0, except for the constant).

George
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