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

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mardi 05 juillet 2011 A  10:25 -0700, Dan Magenheimer a A(C)crit :
> > 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 A  08:54 -0700, Dan Magenheimer a A(C)crit :
> > > 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



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