linux-kernel - Re: wait_on_page_bit_common(TASK_KILLABLE, EXCLUSIVE) can miss wakeup?

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Message-ID: <20200630113637.GC23871@redhat.com>
Date:   Tue, 30 Jun 2020 13:36:37 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>, Jan Kara <jack@...e.cz>,
        Lukas Czerner <lczerner@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: 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.