Subject: Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie

From: William Allen Simpson <[email-masked]>
Date: Thu, 05 Nov 2009 07:45:24 -0500

William Allen Simpson wrote:
> Yes.  Just shuffling the pointers without ever freeing anything.  So,
> there's nothing for call_rcu() to do, and nothing else to synchronize
> (only the pointers).  This assumes that after _unlock_ any CPU cache
> with an old pointer->expires will hit the _lock_ code, and that will
> update *both* ->expires and the other array elements concurrently?
> 
Reiterating, I've not found Documentation showing that this code works:

+	unsigned long jiffy = jiffies;
+
+	if (unlikely(time_after(jiffy, tcp_secret_generating->expires))) {
+		spin_lock_bh(&tcp_secret_locker);
+		if (!time_after(jiffy, tcp_secret_generating->expires)) {
+			/* refreshed by another */
+			spin_unlock_bh(&tcp_secret_locker);
+			memcpy(&xvp->cookie_bakery[0],
+			       &tcp_secret_generating->secrets[0],
+			       sizeof(tcp_secret_generating->secrets));
+		} else {

How is it ensured that an old tcp_secret_generating or an old ->expires,
followed by a spin_lock, has updated both?

And even when both are updated, then every word of the ->secrets array has
also been updated in the local cache?

Is this a property of spin_lock()?  Or spin_unlock()?