linux-kernel - Re: + lock_page_killable-avoid-lost-wakeups.patch added to -mm tree

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Date:	Sat, 17 Jan 2009 22:51:10 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matthew Wilcox <matthew@....cx>,
	Chuck Lever <cel@...i.umich.edu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + lock_page_killable-avoid-lost-wakeups.patch added to -mm tree

I think the patch is correct, just a question,

>  int __lock_page_killable(struct page *page)
>  {
>  	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> +	int ret;
>
> -	return __wait_on_bit_lock(page_waitqueue(page), &wait,
> +	ret = __wait_on_bit_lock(page_waitqueue(page), &wait,
>  					sync_page_killable, TASK_KILLABLE);
> +	/*
> +	 * wait_on_bit_lock uses prepare_to_wait_exclusive, so if multiple
> +	 * procs were waiting on this page, we were the only proc woken up.
> +	 *
> +	 * if ret != 0, we didn't actually get the lock.  We need to
> +	 * make sure any other waiters don't sleep forever.
> +	 */
> +	if (ret)
> +		wake_up_page(page, PG_locked);

This patch assumes that nobody else calls __wait_on_bit_lock() with
action which can return !0. Currently this is correct, but perhaps
it makes sense to move this wake_up_page() into __wait_on_bit_lock ?

Note that we need to "transfer" the wakeup only if wake_up_page()
has already removed us from page_waitqueue(page), this means we
don't need to check ret != 0 twice in __wait_on_bit_lock(), afaics
we can do

	if ((ret = (*action)(q->key.flags))) {
		__wake_up_bit(wq, q->key.flags, q->key.bit_nr);
		// or just __wake_up(wq, TASK_NORMAL, 1, &q->key);
		break;
	}

IOW, imho __wait_on_bit_lock() is buggy, not __lock_page_killable(),
no?

Oleg.

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