On 11/20, Oleg Nesterov wrote:
>
> So, if we have global A == B == 0,
>
> CPU_0 CPU_1
>
> A = 1; B = 2;
> mb(); mb();
> b = B; a = A;
>
> It could happen that a == b == 0, yes? Isn't this contradicts with definition
> of mb?
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.
Oleg.
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