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

[Andrew Morton]

On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 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--;
_