On Tue, 28 Aug 2012 13:09:58 -0700
Eric Dumazet [off-list ref] wrote:

On Tue, 2012-08-28 at 16:17 -0300, Flavio Leitner wrote:

quoted

On Tue, 28 Aug 2012 07:12:34 -0700
Eric Dumazet [off-list ref] wrote:

quoted

On Tue, 2012-08-28 at 21:10 +0800, Wei Yongjun wrote:

quoted

From: Wei Yongjun <redacted>

Remove pointless conditional before kfree_skb().

Signed-off-by: Wei Yongjun <redacted>
---
 include/linux/skbuff.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 7632c87..0b846d9 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -2464,8 +2464,7 @@ static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
 }
 static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
 {
-	if (skb)
-		kfree_skb(skb);
+	kfree_skb(skb);
 }
 #endif
 #ifdef CONFIG_BRIDGE_NETFILTER

Its not exactly pointless.

Its a tradeoff between kernel code size, and ability for cpu to predict
a branch in kfree_skb()

This test is in hot path, and therefore this patch can potentially have
a performance impact.

I really think most kfree_skb() calls are done with a non NULL param,
so the branch prediction is good.

But after this patch, things are totally different.

But then the kfree_skb() is somewhat misleading because it does
check for NULL argument. One would have to remember if it's in
hot path or not. So, what about a new macro to pair with
kfree_skb()? That would document the code and would also
make easier to remember about the performance issue.

For instance:
/* kfree_skb() version to be used in hot code path
 * as the branch prediction can improve performance
 */
#define kfree_skb_hot(skb)	\
	if (skb)		\
		kfree_skb(skb)	\

Really kfree_skb() is not misleading at all :

if (unlikely(!skb))
	return;

So while its _perfectly_ valid to call kfree_skb(NULL), this code
expect callers to not abuse this facility.

Well, I don't think that is obvious. Neither the patch's author.

And nf_conntrack_put_reasm() is called from skb_release_head_state()

We know in this code that most of the time, skb will be NULL.

yeah, but it looks pointless to check the same thing twice.

I dont think we need to add another API for this case and see one
hundred patches coming _trying_ to use this new API.

Ok, and what if kfree_skb() becomes a macro that first checks
if the skb is NULL and if not, call the _kfree_skb() to
continue as before?

#define kfree_skb(skb)		\
        if (skb)		\
		_kfree_skb(skb)	\


void _kfree_skb(struct sk_buff *skb)
{                       
        if (likely(atomic_read(&skb->users) == 1))
                smp_rmb();      
        else if (likely(!atomic_dec_and_test(&skb->users)))
                return;
        trace_kfree_skb(skb, __builtin_return_address(0));
        __kfree_skb(skb);
}

Same API which would work for either use-cases. At the cost of
additional size in the binary.

Just review patches and shout if something bad happens.

I hope we always have you around to catch these cases :)

fbl