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SubjectRe: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss wakeup?
On 06/30, Oleg Nesterov wrote:
>
> On 06/30, Nicholas Piggin wrote:
> >
> > My patch is what actually introduced this ugly
> > bit test, but do we even need it at all? If we do then it's
> > under-commented, I can't see it wouldn't be racy though. Can we just
> > get rid of it entirely?
>
> But then we will need to move io_schedule() down, after test_and_set_bit().
> And we will have the same problem with task->state != RUNNING. Plus more
> complications with "behavior == DROP".

may be someting like this

for (;;) {
int intr = 0;

spin_lock_irq(&q->lock);
if (signal_pending_state(state, current)) {
/* see the comment in prepare_to_wait_event() */
list_del_init(&wait->entry);
intr = 1;
} else {
if (likely(list_empty(&wait->entry))) {
__add_wait_queue_entry_tail(q, wait);
SetPageWaiters(page);
}
set_current_state(state);
}
spin_unlock_irq(&q->lock);

if (behavior == EXCLUSIVE) {
if (!test_and_set_bit_lock(bit_nr, &page->flags))
break;
} else {
int is_set = test_bit(bit_nr, &page->flags);
if (behavior == DROP)
put_page(page);
if (!is_set)
break;
}

if (intr) {
ret = -EINTR;
break;
}

io_schedule();

if (behavior == DROP) {
/*
* We can no longer safely access page->flags:
* even if CONFIG_MEMORY_HOTREMOVE is not enabled,
* there is a risk of waiting forever on a page reused
* for something that keeps it locked indefinitely.
* But best check for -EINTR before breaking.
*/
if (signal_pending_state(state, current))
ret = -EINTR;
break;
}
}

? I dunno...

Oleg.

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