Re: [RFC,PATCH] use rcu for fasync_lock

[Jamie Lokier <jamie@shareable.org>]

Mike Fedyk wrote:
> On Fri, Jan 02, 2004 at 10:41:50PM +0000, Jamie Lokier wrote:
> > The best way is to maintain poll state in each "struct file".  The
> > order of complexity for the bitmap scan is still significant, but
> > ->poll calls are limited to the number of transitions which actually
> > happen.
> 
> What's the drawback to this approach?
> 
> Where is the poll state kept now?

The poll state is not maintained at all _between_ calls to poll/select
at the moment, so at least one fresh call to ->poll is required per
file descriptor.  That is something that can be changed.

> > I think somebody, maybe Richard Gooch, has a patch to do this that's
> > several years old by now.
> 
> Why wasn't it merged?  
> Implementation issues?

The impression I had was that the code is quite complicated and
invasive, and select/poll aren't considered worth optimising because
epoll is an overall better solution (which is true; optimising
select/poll would change the complexity of the slow part but not
reduce the complexity of the API part, while epoll does both).

See ftp://ftp.atnf.csiro.au/pub/people/rgooch/linux/kernel-patches/v2.1/fastpoll-readme

-- Jamie