linux-kernel - Re: Splice on blocking TCP sockets again..

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Message-ID: <20091001221706.GA5191@obsidianresearch.com>
Date:	Thu, 1 Oct 2009 16:17:06 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Volker Lendecke <vl@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: Splice on blocking TCP sockets again..

On Wed, Sep 30, 2009 at 08:00:04AM +0200, Eric Dumazet wrote:

> >> I admit I don't understand why SPLICE_F_NONBLOCK exists, it seems very
> >> un-unixy to have a syscall completely ignore the NONBLOCK flag of the
> >> fd it is called on. Ie setting NONBLOCK on the pipe itself does
> >> nothing when using splice..
> > 
> > Hmm, good question, I dont have the answer but I'll digg one.
> > 
> 
> 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.
> 
> Signed-off-by: Linus Torvalds <torvalds@...l.org>
> 
> See Linus intention was pretty clear : O_NONBLOCK should be taken
> into account by 'actual file that are spliced from/to', regardless
> of SPLICE_F_NONBLOCK flag

Yes, that seems reasonable.

What confuses me is that if O_NONBLOCK is set on the _pipe_ and
SPICE_F_NONBLOCK is not set on the splice call the splice still blocks
- that is unlike other unix apis, eg MSG_DONTWAIT

It seems to me that SPICE_F_NONBLOCK should be or'd with O_NONBLOCK on
the pipe?

Thanks,
Jason
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