Re: [RFC,PATCH 4/14] knfsd: has_wspace per transport

[Greg Banks <gnb@sgi.com>]

On Thu, May 17, 2007 at 08:30:39PM +1000, Neil Brown wrote:
> On Thursday May 17, gnb@sgi.com wrote:
> > On Wed, May 16, 2007 at 05:10:53PM -0400, J. Bruce Fields wrote:
> > > On Thu, May 17, 2007 at 05:22:11AM +1000, Greg Banks wrote:
> > > > +	set_bit(SOCK_NOSPACE, &svsk->sk_sock->flags);
> > > > +	if (required*2 > wspace) {
> > > > +		/* Don't enqueue while not enough space for reply */
> > > > +		dprintk("svc: socket %p  no space, %d*2 > %d, not enqueued\n",
> > > > +			svsk->sk_sk, required, wspace);
> > > > +		return 0;
> > > > +	}
> > > > +	clear_bit(SOCK_NOSPACE, &svsk->sk_sock->flags);
> > > > +	return 1;
> > > > +}
> > > 
> > > So, this is just my ignorance--why do the set and clear of SOCK_NOSPACE
> > > need to be ordered in the way they are?  (Why not just set once inside
> > > the if clause?)
> > 
> > I can't see a good reason for it, but I'm trying to minimise
> > perturbations to the logic.
> 
> Unfortunately, you actually perturbed the important bit... Or at
> least, the bit that I thought was important when I wrote it.
> 
> Previously, sk_stream_wspace(), or sock_wspace() would be called *after*
> SOCK_NOSPACE was set.  With your patch it is called *before*.

Latest patch does this:

+/**
+ * svc_udp_has_wspace - Checks if there is enough space
+ * to send the reply on the socket without blocking.
+ * @svsk: the svc_sock to write on
+ * @required: the number of bytes which should be available
+ */
+static int
+svc_udp_has_wspace(struct svc_sock *svsk, int required)
+{
+	int wspace;
+
+	/*
+	 * Neil Brown explains the gymnastics with SOCK_NOSPACE thus:
+	 * It is a fairly improbably race, but if the output queue
+	 * flushed completely between calling sk_stream_wspace() or
+	 * sock_wspace() and setting SOCK_NOSPACE, the sk_write_space
+	 * callback might never get called.
+	 */
+	set_bit(SOCK_NOSPACE, &svsk->sk_sock->flags);
+	wspace = sock_wspace(svsk->sk_sk);
+	if (required*2 > wspace) {
+		/* Don't enqueue while not enough space for reply */
+		dprintk("svc: socket %p  no space, %d*2 > %d, not enqueued\n",
+			svsk->sk_sk, required, wspace);
+		return 0;
+	}
+	clear_bit(SOCK_NOSPACE, &svsk->sk_sock->flags);
+	return 1;
+}
+

And the same gymnastics are repeated in svc_tcp_has_wspace() which
now has no common code with the UDP case.  I also calculate `required'
in generic code and pass it in.

Greg.
-- 
Greg Banks, R&D Software Engineer, SGI Australian Software Group.
Apparently, I'm Bedevere.  Which MPHG character are you?
I don't speak for SGI.

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