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

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Date:	Tue, 5 Jul 2011 12:07:10 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Konrad Wilk <konrad.wilk@...cle.com>,
	linux-mm <linux-mm@...ck.org>
Subject: RE: [RFC] non-preemptible kernel socket for RAMster

> > > > +++ 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

Oh, OK.  My use model is that a socket that is used non-preemptible
must always be used non-preemptible.  In other words, this kind
of socket is an extreme form of non-blocking.  Doesn't that seem
like a reasonable constraint? 

Thanks,
Dan
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