netdev - Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect for sockets in accept(3)

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Date:	Thu, 22 Oct 2015 04:30:25 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Casper.Dik@...cle.com
Cc:	Alan Burlison <Alan.Burlison@...cle.com>,
	David Miller <davem@...emloft.net>, viro@...IV.linux.org.uk,
	stephen@...workplumber.org, netdev@...r.kernel.org,
	dholland-tech@...bsd.org
Subject: Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect
 for sockets in accept(3)

On Thu, 2015-10-22 at 08:15 +0200, Casper.Dik@...cle.com wrote:
> >It's been said that the current mechanisms in Linux & some BSD variants 
> >can be subject to races, and the behaviour exhibited doesn't conform to 
> >POSIX, for example requiring the use of shutdown() on unconnected 
> >sockets because close() doesn't kick off other threads accept()ing on 
> >the same fd. I'd be interested to hear if there's a better and more 
> >performant way of handling the situation that doesn't involve doing the 
> >sort of bookkeeping Casper described,.
> 
> Of course, the implementation is now around 18 years old; clearly a lot of 
> things have changed since then.
> 
> In the particular case of Linux close() on a socket, surely it must be 
> possible to detect at close that it is a listening socket and that you are 
> about to close the last reference; the kernel could then do the shutdown() 
> all by itself.

We absolutely do not _want_ to do this just so that linux becomes slower
to the point Solaris can compete, or you guys can avoid some work.

close(fd) is very far from knowing a file is a 'listener' or even a
'socket' without extra cache line misses.

To force a close of an accept() or whatever blocking socket related
system call a shutdown() makes a lot of sense.

This would have zero additional overhead for the fast path.


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