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.