RE: [RFC] non-preemptible kernel socket for RAMster

Le mardi 05 juillet 2011 à 10:25 -0700, Dan Magenheimer a écrit :

quoted

From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
Sent: Tuesday, July 05, 2011 10:31 AM
To: Dan Magenheimer
Cc: netdev@vger.kernel.org; Konrad Wilk; linux-mm
Subject: Re: [RFC] non-preemptible kernel socket for RAMster

Le mardi 05 juillet 2011 à 08:54 -0700, Dan Magenheimer a écrit :

quoted

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;
+	}

Hmm, was this tested on UP machine ?

Hi Eric --

Thanks for the reply!

I hadn't tested UP in awhile so am testing now, and it seems to
work OK so far.  However, I am just testing my socket, *not* testing
sockets in general.  Are you implying that this patch will
break (kernel) sockets in general on a UP machine?  If so,
could you be more specific as to why?  (Again, I said
I am a networking idiot. ;-)  I played a bit with adding
a new SOCK_ flag and triggering off of that, but this
version of the patch seemed much simpler.

Say you have two processes and socket S

One process locks socket S, and is preempted by another process.

This second process is non preemptible and try to lock same socket.

-> deadlock, since P1 never releases socket S