Messages in this thread Patch in this message | | | Date | Thu, 30 Nov 2017 12:47:36 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock |
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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--; _
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