Re: [PATCH v3] net-tcp: TCP/IP stack bypass for loopback connections

On Mon, 2012-09-17 at 11:58 -0700, Bruce "Brutus" Curtis wrote:

From: "Bruce \"Brutus\" Curtis" <redacted>

TCP/IP loopback socket pair stack bypass, based on an idea by, and
rough upstream patch from, David Miller [off-list ref] called
"friends", the data structure modifcations and connection scheme are
reused with extensive data-path changes.

...

 
+		if (skb->friend) {
+			/*
+			 * If friends haven't been made yet, our sk_friend
+			 * still == NULL, then update with the ACK's friend
+			 * value (the listen()er's sock addr) which is used
+			 * as a place holder.
+			 */
+			cmpxchg(&sk->sk_friend, NULL, skb->friend);
+		}

There is a fundamental issue with this patch

Setting skb->friend to a socket structure, without holding a reference
on it is going to add subtle races and bugs.

In this code, we have no guarantee the socket pointed by skb->friend was
eventually freed and/or reused.

But adding references might be overkill, as we need to unref them in
some places, in hot path.