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

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Message-Id: <20140401.121553.1464672258686459428.davem@davemloft.net>
Date:	Tue, 01 Apr 2014 12:15:53 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ebiederm@...ssion.com
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

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Tue, 01 Apr 2014 01:03:53 -0700

> 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.

Fair enough.
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