netdev - [RFC] non-preemptible kernel socket for RAMster

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Date:	Tue, 5 Jul 2011 08:54:06 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	netdev@...r.kernel.org
Cc:	Konrad Wilk <konrad.wilk@...cle.com>, linux-mm <linux-mm@...ck.org>
Subject: [RFC] non-preemptible kernel socket for RAMster

In working on a kernel project called RAMster* (where RAM on a
remote system may be used for clean page cache pages and for swap
pages), I found I have need for a kernel socket to be used when
in non-preemptible state.  I admit to being a networking idiot,
but I have been successfully using the following small patch.
I'm not sure whether I am lucky so far... perhaps more
sockets or larger/different loads will require a lot more
changes (or maybe even make my objective impossible).
So I thought I'd post it for comment.  I'd appreciate
any thoughts or suggestions.

Thanks,
Dan

* http://events.linuxfoundation.org/events/linuxcon/magenheimer 

diff -Napur linux-2.6.37/net/core/sock.c linux-2.6.37-ramster/net/core/sock.c
--- linux-2.6.37/net/core/sock.c	2011-07-03 19:14:52.267853088 -0600
+++ linux-2.6.37-ramster/net/core/sock.c	2011-07-03 19:10:04.340980799 -0600
@@ -1587,6 +1587,14 @@ static void __lock_sock(struct sock *sk)
 	__acquires(&sk->sk_lock.slock)
 {
 	DEFINE_WAIT(wait);
+	if (!preemptible()) {
+		while (sock_owned_by_user(sk)) {
+			spin_unlock_bh(&sk->sk_lock.slock);
+			cpu_relax();
+			spin_lock_bh(&sk->sk_lock.slock);
+		}
+		return;
+	}
 
 	for (;;) {
 		prepare_to_wait_exclusive(&sk->sk_lock.wq, &wait,
@@ -1623,7 +1631,8 @@ static void __release_sock(struct sock *
 			 * This is safe to do because we've taken the backlog
 			 * queue private:
 			 */
-			cond_resched_softirq();
+			if (preemptible())
+				cond_resched_softirq();
 			skb = next;
 		} while (skb != NULL);
--
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