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

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Message-ID: <4AF2C8E4.9020202@gmail.com>
Date:	Thu, 05 Nov 2009 07:45:24 -0500
From:	William Allen Simpson <william.allen.simpson@...il.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Linux Kernel Developers <linux-kernel@...r.kernel.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder	Cookie

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()?
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