From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect
 for sockets in accept(3)
Date: Sat, 31 Oct 2015 21:51:16 +0000
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Cc: Eric Dumazet <eric.dumazet@gmail.com>,
	David Miller <davem@davemloft.net>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Network Development <netdev@vger.kernel.org>,
	David Howells <dhowells@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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On Sat, Oct 31, 2015 at 02:23:31PM -0700, Linus Torvalds wrote:

> The other stuff we probably can't do all that much about. Unless we
> decide to go for some complicated lockless optimistic file descriptor
> allocation scheme with retry-on-failure instead of locks. Which I'm
> sure is possible, but I'm equally sure is painful.

The interesting part is dup2() - we'd have to do something like
	serialize against other dup2
	was_claimed = atomically set and test bit in bitmap
	if was_claimed
		tofree = fdt->fd[fd];
		if (!tofree)
			fail with EBUSY
	install into ->fd[...]
	end of critical area
in there; __alloc_fd() could be made retry-on-failure, but I don't see
how to cope with dup2 vs. dup2 without an explicit exclusion.