netdev - Re: [PATCH 1/3] net: Add a test to see if a skb is freeable in irq context

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Date:	Tue, 01 Apr 2014 01:03:53 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	David Miller <davem@...emloft.net>
Cc:	bjorn@...k.no, eric.dumazet@...il.com, ben@...adent.org.uk,
	stephen@...workplumber.org, netdev@...r.kernel.org,
	xiyou.wangcong@...il.com, mpm@...enic.com, satyam.sharma@...il.com,
	David.Laight@...LAB.COM
Subject: Re: [PATCH 1/3] net: Add a test to see if a skb is freeable in irq context

David Miller <davem@...emloft.net> writes:

> From: ebiederm@...ssion.com (Eric W. Biederman)
> Date: Thu, 27 Mar 2014 18:15:47 -0700
>
>> Currently netpoll and skb_release_head_state assume that a skb is
>> freeable in hard irq context except when skb->destructor is set.
>> 
>> The reality is far from this.  So add a function skb_irq_freeable to
>> compute the full test and in the process be the living documentation of
>> what the requirements are of actually freeing a skb in hard irq context.
>> 
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>  ...
>> +	return !skb->destructor &&
>> +#if IS_ENABLED(CONFIG_XFRM)
>> +		!skb->sp &&
>> +#endif
>> +#if IS_ENABLED(CONFIG_NF_CONNTRACK)
>> +		!skb->nfct &&
>> +#endif
>> +		!skb->_skb_refdst &&
>> +		!skb_has_frag_list(skb);
>
> I think you need to add "!skb->nf_bridge &&" to this test.

Given that the definition of nf_bridge_put is just:

static inline void nf_bridge_put(struct nf_bridge_info *nf_bridge)
{
	if (nf_bridge && atomic_dec_and_test(&nf_bridge->use))
		kfree(nf_bridge);
}

I don't see why.

atomic_dec_and_test and kfree are hard irq safe.

I can see the code evolving in a way where it wouldn't be safe to put a
nf_bridge from hard irq context but the code as it is today is trivially
safe.

Eric

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