netdev - Re: Splice on blocking TCP sockets again..

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Date:	Fri, 2 Oct 2009 11:10:29 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Volker Lendecke <Volker.Lendecke@...Net.DE>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	Volker Lendecke <vl@...ba.org>
Subject: Re: Splice on blocking TCP sockets again..

On Wed, Sep 30, 2009 at 08:37:13AM +0200, Volker Lendecke wrote:
> On Tue, Sep 29, 2009 at 06:48:20PM -0600, Jason Gunthorpe wrote:
> > FWIW, it looks like samba has a splice code now, but doesn't enable it
> > due to this issue?
> 
> Right. What I've learned from the comments is that splice is
> only usable in multi-threaded programs. One thread is
> reading, one is writing from the other end. I deferred using
> splice until we have the proper architecture to do sync
> syscalls in helper threads to make them virtually async.  We
> have some code for that now, but it's not a high priority
> for me at this moment.

So, it looks like thanks to Eric and davem that splice will be changed
so it can be blocking on the TCP and non-blocking on the PIPE.

I'd suggest a construct like the following as a compatability
solution:

struct pollfd pfd = {.fd = tcpfd, events = POLLIN | POLLRDHUP};
while (..) {
   rc = splice(tcpfd,0,pfd[1],0,count,SPLICE_F_MOVE | SPLICE_F_NONBLOCK);
   if (rc == -1)
     //...
   if (rc == 0) {
       if (pfd.revents & POLLRDHUP)
          // oops, EOF on TCP

       /* Might be an old kernel that nonblocks on TCP, have to check
          if this is EOF or do blocking. */
       rc = poll(&pfd,1,-1);
       if (rc == -1)
          //...
   }

   rc = splice(pfd[0],0,ofd,0,..., SPLICE_F_MOVE)
}

Which should add no overhead in the new splice blocks case, and falls
back gracefully on older kernels..

Thanks,
Jason
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