netdev - Re: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O

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Date:	Thu, 17 Jul 2008 21:41:56 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Octavian Purdila <opurdila@...acom.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O_NONBLOCK

On Thu, Jul 17, 2008 at 05:47:27PM +0300, Octavian Purdila (opurdila@...acom.com) wrote:
> > Existing behaviour was selected to be able to have a progress if socket
> > does not have enough data to fill the pipe. With your change if socket
> > is not opened with non-blocking mode reading will block not matter if
> > SPLICE_F_NONBLOCK is set or not. 
> 
> I am probably missing some usecases here, but usually if you want to use 
> non-blocking I/O you need to use special approach anyway (e.g. code the 
> poll/epoll/select bits) so then you could open the socket with O_NONBLOCK.

It depends. Splice clearly states that it tries to be nonblocking with
given flag being set, and its reading will be non-blocking indeed.

> > This is a quite serious break of the 
> > overall idea behind SPLICE_F_NONBLOCK.
> >
> 
> I don't know... the man page explicitly says that even when you use 
> SPLICE_F_NONBLOCK splice may block because of the underlying fd blocking.

Yes, but reading from the network will not.

> But more importantly, how can we solve the deadlock issue described in the 
> patch? Do we need all of the complications of async I/O for such a simple and 
> common usecase?

I'm not sure I understand how it can deadlock, please explain it in more
details.

-- 
	Evgeniy Polyakov
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