netdev - Re: [PATCH 6/6] net: Free skbs from irqs when possible.

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Date:	Tue, 18 Mar 2014 06:22:36 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	David Miller <davem@...emloft.net>, stephen@...workplumber.org,
	netdev@...r.kernel.org, xiyou.wangcong@...il.com, mpm@...enic.com,
	satyam.sharma@...il.com
Subject: Re: [PATCH 6/6] net: Free skbs from irqs when possible.

On Mon, 2014-03-17 at 23:27 -0700, Eric W. Biederman wrote:
> Add a test skb_irq_freeable to report when it is safe to free a skb
> from irq context.
> 
> It is not safe to free an skb from irq context when:
> - The skb has a destructor as some skb destructors call local_bh_disable
>   or spin_lock_bh.
> - There is xfrm state as __xfrm_state_destroy calls spin_lock_bh.
> - There is netfilter conntrack state as destroy_conntrack calls
>   spin_lock_bh.
> - If there is a refcounted dst entry on the skb, as __dst_free
>   calls spin_lock_bh.
> - If there is a frag_list, which could be a list of any skbs.
> Otherwise it appears safe to free a skb from interrupt context.
> 
> - Update the warning in skb_releae_head_state to warn about freeing
>   skb's in the wrong context.
> 
> - Update __dev_kfree_skb_irq to free all skbs that it can immediately
> 
> - Kill zap_completion_queue because there is no point going through
>   a queue of packets that are not safe to free and looking for packets
>   that are safe to free.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  include/linux/skbuff.h |   13 +++++++++++++
>  net/core/dev.c         |   14 +++++++++-----
>  net/core/netpoll.c     |   32 --------------------------------
>  net/core/skbuff.c      |   13 ++++++++++---
>  4 files changed, 32 insertions(+), 40 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 03db95ab8a8c..53f72b53fd47 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2833,6 +2833,19 @@ static inline void skb_init_secmark(struct sk_buff *skb)
>  { }
>  #endif
>  
> +static inline bool skb_irq_freeable(struct sk_buff *skb)
> +{
> +	return !skb->destructor &&
> +#if IS_ENABLED(CONFIG_XFRM)
> +		!skb->sp &&
> +#endif
> +#if IS_ENABLED(CONFIG_NF_CONNTRACK)
> +		!skb->nfct &&
> +#endif
> +		(!skb->_skb_refdst || (skb->_skb_refdst & SKB_DST_NOREF)) &&
> +		!skb_has_frag_list(skb);
> +}
> +

It would be a serious bug having (skb->_skb_refdst & SKB_DST_NOREF) at
this point. dst would be RCU protected, but this can not be true as the
packet was queued in TX ring buffer for a possibly long period. And even
before reaching the driver, skb might have been queued in qdisc layer
and escape rcu protection section anyway.

Thats why we use skb_dst_force() from __dev_xmit_skb()



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