linux-kernel - Re: [RFC PATCH] mm: silence soft lockups from unlock

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Date:   Fri, 24 Jul 2020 16:45:25 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page

On 07/23, Linus Torvalds wrote:
>
> IOW, I think we should do something like this (this is on top of my
> patch, since it has that wake_page_function() change in it, but notice
> how we have the exact same issue in our traditional
> autoremove_wake_function() usage).

...

> +static inline void list_del_init_careful(struct list_head *entry)
> +{
> +	__list_del_entry(entry);
> +	entry->prev = entry;
> +	smp_store_release(&entry->next, entry);
> +}
> +
...
>  static inline int list_empty_careful(const struct list_head *head)
>  {
> -	struct list_head *next = head->next;
> +	struct list_head *next = smp_load_acquire(&head->next);
>  	return (next == head) && (next == head->prev);
>  }

This (and your previous email) answers my concerns about memory barriers.

IIUC, finish_wait() could even use this version of list_empty_careful(),

	struct list_head *next = smp_load_acquire(&head->next);
	return (next == head) && !WARN_ON(next != head->prev);

iow, it doesn't really need to check next == head->prev as long as only
list_del_init_careful() can remove it from list.

Thanks!

Oleg.