Re: [PATCH 3/3] net: skb_queue_purge(): lock/unlock the list only once

Stephen Hemminger
2017-09-08T16:51:34Z

On Fri, 08 Sep 2017 05:06:30 -0000
Michael Witten <mfwitten@gmail.com> wrote:

> Date: Thu, 7 Sep 2017 20:07:40 +0000
> With this commit, the list's lock is locked/unlocked only once
> for the duration of `skb_queue_purge()'.
>
> Hitherto, the list's lock has been locked/unlocked every time
> an item is dequeued; this seems not only inefficient, but also
> incorrect, as the whole point of `skb_queue_purge()' is to clear
> the list, presumably without giving anything else a chance to
> manipulate the list in the interim.
>
> Signed-off-by: Michael Witten <mfwitten@gmail.com>
> ---
> net/core/skbuff.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 68065d7d383f..66c0731a2a5f 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2834,9 +2834,13 @@ EXPORT_SYMBOL(skb_dequeue_tail);
> */
> void skb_queue_purge(struct sk_buff_head *list)
> {
> + unsigned long flags;
> struct sk_buff *skb;
> - while ((skb = skb_dequeue(list)) != NULL)
> +
> + spin_lock_irqsave(&list->lock, flags);
> + while ((skb = __skb_dequeue(list)) != NULL)
> kfree_skb(skb);
> + spin_unlock_irqrestore(&list->lock, flags);
> }
> EXPORT_SYMBOL(skb_queue_purge);
>

As Eric said, this won't work.

Instead why not introduce something list splice which moves next/prev
of list head to a local list on the stack.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 68065d7d383f..4988b6efdcc8 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2824,6 +2824,44 @@ struct sk_buff *skb_dequeue_tail(struct sk_buff_head *list)
}
EXPORT_SYMBOL(skb_dequeue_tail);

+static void __skb_splice(const struct sk_buff_head *list,
+ struct sk_buff *prev,
+ struct sk_buff *next)
+{
+ struct sk_buff *first = list->next;
+ struct sk_buff *last = list->prev;
+
+ list->qlen = 0;
+
+ first->prev = prev;
+ prev->next = first;
+
+ list->next = next;
+ next->prev = last;
+}
+
+/**
+ * skb_splice - join two lists, and initialize the emptied list
+ * @list: the new list to add
+ * @head: the pace to add it in the first list
+ *
+ * Take the first list (@list) and merge it onto the
+ * head of existing list (@head).
+ */
+static void skb_splice_init(const struct sk_buff_head *list,
+ struct sk_buff_head *head)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&list->lock, flags);
+ if (list->qlen > 0) {
+ head->qlen += list->qlen;
+ __skb_splice(list, head, head->next);
+ __skb_queue_head_init(list);
+ }
+ spin_unlock_irqrestore(&list->lock, flags);
+}
+
/**
* skb_queue_purge - empty a list
* @list: list to empty
@@ -2835,7 +2873,12 @@ EXPORT_SYMBOL(skb_dequeue_tail);
void skb_queue_purge(struct sk_buff_head *list)
{
struct sk_buff *skb;
- while ((skb = skb_dequeue(list)) != NULL)
+ struct skb_buff_head tmp;
+
+ __skb_queue_head_init(&tmp);
+ skb_splice_init(list, &tmp);
+
+ while ((skb = __skb_dequeue(list)) != NULL)
kfree_skb(skb);
}
EXPORT_SYMBOL(skb_queue_purge);