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Date:   Thu, 23 Jul 2020 20:01:00 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Hugh Dickins <hughd@google.com>, Michal Hocko <mhocko@kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Michal Hocko <mhocko@suse.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page
Message-ID: <20200723180100.GA21755@redhat.com>
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On 07/23, Linus Torvalds wrote:
>
> So here's a v2, now as a "real" commit with a commit message and everything.

I am already sleeping, will read it tomorrow, but at first glance...

> @@ -1013,18 +1014,40 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
>  	if (wait_page->bit_nr != key->bit_nr)
>  		return 0;
>  
> +	/* Stop walking if it's locked */
> +	if (wait->flags & WQ_FLAG_EXCLUSIVE) {
> +		if (test_and_set_bit(key->bit_nr, &key->page->flags))
> +			return -1;
> +	} else {
> +		if (test_bit(key->bit_nr, &key->page->flags))
> +			return -1;
> +	}
> +
>  	/*
> -	 * Stop walking if it's locked.
> -	 * Is this safe if put_and_wait_on_page_locked() is in use?
> -	 * Yes: the waker must hold a reference to this page, and if PG_locked
> -	 * has now already been set by another task, that task must also hold
> -	 * a reference to the *same usage* of this page; so there is no need
> -	 * to walk on to wake even the put_and_wait_on_page_locked() callers.
> +	 * Let the waiter know we have done the page flag
> +	 * handling for it (and the return value lets the
> +	 * wakeup logic count exclusive wakeup events).
>  	 */
> -	if (test_bit(key->bit_nr, &key->page->flags))
> -		return -1;
> +	ret = (wait->flags & WQ_FLAG_EXCLUSIVE) != 0;
> +	wait->flags |= WQ_FLAG_WOKEN;
> +	wake_up_state(wait->private, mode);
>  
> -	return autoremove_wake_function(wait, mode, sync, key);
> +	/*
> +	 * Ok, we have successfully done what we're waiting for,
> +	 * and we can unconditionally remove the wait entry.
> +	 *
> +	 * Note that this has to be the absolute last thing we do,
> +	 * since after list_del_init(&wait->entry) the wait entry
> +	 * might be de-allocated and the process might even have
> +	 * exited.
> +	 *
> +	 * We _really_ should have a "list_del_init_careful()" to
> +	 * properly pair with the unlocked "list_empty_careful()"
> +	 * in finish_wait().
> +	 */
> +	smp_mb();
> +	list_del_init(&wait->entry);

I think smp_wmb() would be enough, but this is minor.

We need a barrier between "wait->flags |= WQ_FLAG_WOKEN" and list_del_init(),

But afaics we need another barrier, rmb(), in wait_on_page_bit_common() for
the case when wait->private was not blocked; we need to ensure that if
finish_wait() sees list_empty_careful() == T then we can't miss WQ_FLAG_WOKEN.

Oleg.