Re: Unifying epoll,aio,futexes etc. (What I really want from epoll)

[Rusty Russell <rusty@rustcorp.com.au>]

On 31 Oct 2002 16:45:58 +0000
Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> What is hard is multiple futex waits and livelock for that. I think it
> can be done properly but I've not sat down and designed it all out - I
> wonder what Rusty thinks.

Hmm... Never thought about it.  You mean an API like:

	struct futex_set *futex_set_init();
	struct futex_set *futex_set_add(struct futex_set *, struct futex *);
	/* Returns futex obtained. */
	struct futex *futex_set_wait(struct futex_set *);

I think a naive implementation of futex_set_wait would look like:

	set = futex_set
 try:
	for each futex in set {
		if (grab in userspace) {
			close fds;
			return with futex;
		}
		close old fd for futex if any
		call FUTEX_FD to get fd notification of futex;
	}

	select on fds
	set = fds which are ready
	goto try

You could, of course, loop through the fast path once before making any
syscalls.  Another optimization is to have FUTEX_FD reuse an existing fd
rather than requiring the close.

Not sure I get the point about livelock though: deadlock is possible if
apps seek multiple locks at once without care, of course.

Rusty.
-- 
   there are those who do and those who hang on and you don't see too
   many doers quoting their contemporaries.  -- Larry McVoy