netdev - Re: SPLICE_F_NONBLOCK semantics...

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Date:	Thu, 1 Oct 2009 15:21:44 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
cc:	eric.dumazet@...il.com, jgunthorpe@...idianresearch.com,
	vl@...ba.org, opurdila@...acom.com, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: SPLICE_F_NONBLOCK semantics...



On Thu, 1 Oct 2009, David Miller wrote:
> 
> It depends upon our interpretation of how you intended the
> SPLICE_F_NONBLOCK flag to work when you added it way back
> when.
> 
> Linus introduced  SPLICE_F_NONBLOCK in commit 29e350944fdc2dfca102500790d8ad6d6ff4f69d
> (splice: add SPLICE_F_NONBLOCK flag )
> 
>   It doesn't make the splice itself necessarily nonblocking (because the
>   actual file descriptors that are spliced from/to may block unless they
>   have the O_NONBLOCK flag set), but it makes the splice pipe operations
>   nonblocking.
> 
> Linus intention was clear : let SPLICE_F_NONBLOCK control the splice pipe mode only

Ack. The original intent was for the flag to affect the buffering, not the 
end points.

> splice(socket,0,pipe,0,128*1024,SPLICE_F_MOVE | SPLICE_F_NONBLOCK ); 
> 
> to block on data coming from socket (if file is in blocking mode),
> and not block on pipe output (to avoid deadlock)

Yes. Sounds correct. Although the more I think about it, the more I 
suspect that the whole NONBLOCK thing should probably have been two bits, 
and simply been about "nonblocking input" vs "nonblocking output" (so that 
you could control both sides on a call-by-call basis).

But I think that your patch is fundamentally correct with the semantics 
as-is.

		Linus
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