Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock

[Waiman Long <longman@redhat.com>]

On 11/30/2017 03:47 PM, Andrew Morton wrote:
> On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long <longman@redhat.com> wrote:
>
>>> And, from that perspective, the racy shortcut in the proposed patch
>>> is wrong, too. Prefetch is fine, but in general shortcutting list
>>> empty checks outside the internal lock isn't.
>> For the record, I add one more list_empty() check at the beginning of
>> list_lru_del() in the patch for 2 purpose:
>> 1. it allows the code to bail out early.
>> 2. It make sure the cacheline of the list_head entry itself is loaded.
>>
>> Other than that, I only add a likely() qualifier to the existing
>> list_empty() check within the lock critical region.
> But it sounds like Dave thinks that unlocked check should be removed?
>
> How does this adendum look?
>
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix
>
> include prefetch.h, remove unlocked list_empty() test, per Dave
>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Waiman Long <longman@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
>  mm/list_lru.c |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff -puN mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix mm/list_lru.c
> --- a/mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix
> +++ a/mm/list_lru.c
> @@ -8,6 +8,7 @@
>  #include <linux/module.h>
>  #include <linux/mm.h>
>  #include <linux/list_lru.h>
> +#include <linux/prefetch.h>
>  #include <linux/slab.h>
>  #include <linux/mutex.h>
>  #include <linux/memcontrol.h>
> @@ -135,13 +136,11 @@ bool list_lru_del(struct list_lru *lru,
>  	/*
>  	 * Prefetch the neighboring list entries to reduce lock hold time.
>  	 */
> -	if (unlikely(list_empty(item)))
> -		return false;
>  	prefetchw(item->prev);
>  	prefetchw(item->next);
>  
>  	spin_lock(&nlru->lock);
> -	if (likely(!list_empty(item))) {
> +	if (!list_empty(item)) {
>  		l = list_lru_from_kmem(nlru, item);
>  		list_del_init(item);
>  		l->nr_items--;
> _
>
Yes, that look good to me.

Thanks for fixing that.

Cheers,
Longman

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