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

Eric Dumazet
2017-09-08T16:01:25Z

On Fri, 2017-09-08 at 05:06 +0000, Michael Witten 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);
>

No, this is very wrong :

Holding hard IRQ for a potential very long time is going to break
horribly. Some lists can have 10,000+ skbs in them.

Note that net-next tree is currently closed, please read
Documentation/networking/netdev-FAQ.txt