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

From: William Allen Simpson
Date: Thu Nov 05 2009 - 07:45:40 EST

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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