netdev - Re: [PATCH] GRE: Use IS_ERR_OR_NULL in gre_gso

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Date:	Sun, 21 Apr 2013 21:44:33 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	horms@...ge.net.au
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org, xeb@...l.ru
Subject: Re: [PATCH] GRE: Use IS_ERR_OR_NULL in gre_gso_segment

From: Simon Horman <horms@...ge.net.au>
Date: Mon, 22 Apr 2013 10:35:57 +0900

> On Fri, Apr 19, 2013 at 02:28:52PM -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Fri, 19 Apr 2013 03:24:33 -0700
>> 
>> > On Fri, 2013-04-19 at 15:48 +0900, Simon Horman wrote:
>> >> Signed-off-by: Simon Horman <horms@...ge.net.au>
>> >> ---
>> >>  net/ipv4/gre.c |    2 +-
>> >>  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> 
>> >> diff --git a/net/ipv4/gre.c b/net/ipv4/gre.c
>> >> index d2d5a99..0ae998b 100644
>> >> --- a/net/ipv4/gre.c
>> >> +++ b/net/ipv4/gre.c
>> >> @@ -168,7 +168,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>> >>  	/* segment inner packet. */
>> >>  	enc_features = skb->dev->hw_enc_features & netif_skb_features(skb);
>> >>  	segs = skb_mac_gso_segment(skb, enc_features);
>> >> -	if (!segs || IS_ERR(segs))
>> >> +	if (IS_ERR_OR_NULL(segs))
>> >>  		goto out;
>> >>  
>> >>  	skb = segs;
>> > 
>> > Hi Simon
>> > 
>> > AFAIK I would change things so that NULL is not a possible value.
>> > 
>> > I don't really like IS_ERR_OR_NULL() because it hides some lazyness of
>> > ours, and is more expensive (2 tests)
>> > 
>> > If we return NULL for an error, why not instead return -Esomething,
>> > since caller is OK to get -ENOMEM,-Exxxxx,... ?
>> 
>> Sometimes IS_ERR_OR_NULL is appropriate, but not here, since the caller
>> can more easily just provide good error codes all the time instead of
>> sometimes returning nULL.
> 
> I am confused.
> 
> I'm not sure that my change actually alters the logic at all.
> Is the suggestion that the logic should be changed somehow?

We're saying change skb_mac_gso_segment() to never return NULL, and
always an error encoded pointer, rather than change the callers.
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