netdev - [PATCH] net: avoid one atomic op per cloned skb

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Date:	Tue, 18 May 2010 15:40:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>
Subject: [PATCH] net: avoid one atomic op per cloned skb

Hi David

I know you said 'only patches', but I found following patch small
enough ?

I have a followup patch to avoid two atomic ops per cloned skb on
dataref (helps TCP tx path) but will submit it for 2.6.36, since its
diffstat is a bit more than 3++- :)

Thanks

[PATCH] net: avoid one atomic op per cloned skb

skb_clone() can use atomic_set(clone_ref, 2) safely, because only
current thread can possibly touch clone_ref at this point.
   
Add a WARN_ON_ONCE() for a while, to catch wrong assumptions.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 net/core/skbuff.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c543dd2..4444f15 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -628,7 +628,8 @@ struct sk_buff *skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
 	    n->fclone == SKB_FCLONE_UNAVAILABLE) {
 		atomic_t *fclone_ref = (atomic_t *) (n + 1);
 		n->fclone = SKB_FCLONE_CLONE;
-		atomic_inc(fclone_ref);
+		WARN_ON_ONCE(atomic_read(fclone_ref) != 1);
+		atomic_set(fclone_ref, 2);
 	} else {
 		n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
 		if (!n)



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