2008-05-07 16:26:15

by J. Bruce Fields

[permalink] [raw]
Subject: Re: [PATCH 1/3] svcrdma: Remove redunant call to xpo_release_rqst method.

On Wed, May 07, 2008 at 10:08:06AM -0500, Tom Tucker wrote:
> On Tue, 2008-05-06 at 17:59 -0400, J. Bruce Fields wrote:
> > On Fri, May 02, 2008 at 11:18:04AM -0500, Tom Tucker wrote:
> > > The xpo_release_rqst method is called from svc_xprt_release and therefore
> > > this direct call of the provider method is redundant. There is no bug
> > > here since the provider protects against multiple calls.
> > >
> > > Originally, this code was here because the xpo_release_rqst function was
> > > being used by the RDMA transport to return credits to the client, i.e.
> > > posting a receive buffer. This had to be done before the send in order to
> > > avoid a race wherein the client responds immediately with a new request
> > > but the buffer has not yet been posted. This is a poor design point and
> > > the recv buffer posting has been modified in the rdma transport.
> >
> > The comment there refers to the socket code, not the rdma code, and
> Yes, you are correct.
> > there was a svc_release_skb() there before the rdma code came along.
> > I'd like to understand why.
> I believe it's because the socket code has buffer quotas and the skb
> you're holding consumes credits in the recv sock buf. So it's
> conceivable that you could send a message, another client message
> arrives prior to releasing the skb your holding, and this new message
> gets dropped unnecessarily. I don't believe it affects the sending at
> all because the message you're holding came from sock_recv_datagram and
> only consumes recv side credits.
> Since this patch is only for "clean up" purposes, I'm inclined to just
> ditch this patch and let us noodle on it a little more. Agreed?

Yep, sounds prudent.