On Mon, Nov 27, 2006 at 04:10:27PM -0500, Alan Stern wrote:
> On Mon, 27 Nov 2006, Oleg Nesterov wrote:
>
> > I still can't relax, another attempt to "prove" this should not be
> > possible on CPUs supported by Linux :)
> >
> > Let's suppose it is possible, then it should also be possible if CPU_1
> > does spin_lock() instead of mb() (spin_lock can't be "stronger"), yes?
> >
> > Now,
> >
> > int COND;
> > wait_queue_head_t wq;
> >
> > my_wait()
> > {
> > add_wait_queue(&wq);
> > for (;;) {
> > set_current_state(TASK_UNINTERRUPTIBLE);
> >
> > if (COND)
> > break;
> >
> > schedule();
> > }
> > remove_wait_queue(&wq);
> > }
> >
> > my_wake()
> > {
> > COND = 1;
> > wake_up(&wq);
> > }
> >
> > this should be correct, but it is not!
> >
> > my_wait:
> >
> > task->state = TASK_UNINTERRUPTIBLE; // STORE
> >
> > mb();
> >
> > if (COND) break; // LOAD
> >
> >
> > my_wake:
> >
> > COND = 1; // STORE
> >
> > spin_lock(WQ.lock);
> > spin_lock(runqueue.lock);
> >
> > // try_to_wake_up()
> > if (!(task->state & TASK_UNINTERRUPTIBLE)) // LOAD
> > goto out;
> >
> >
> > So, my_wait() gets COND == 0, and goes to schedule in 'D' state.
> > try_to_wake_up() reads ->state == TASK_RUNNING, and does nothing.
>
> This is a very good point. I don't know what the resolution is; Paul will
> have to explain the situation.
I am revisiting this, and will let you know what I learn.
Thanx, Paul
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