Re: [RFC,PATCH] use rcu for fasync_lock

Ingo Oeser
2004-01-04T19:03:57Z

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On Saturday 03 January 2004 22:28, Jamie Lokier wrote:
> 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.

Yes, file->f_mode can be hijacked for this. Only 2 bits of it are used at the
moment. More headache is clearing this state again, but this might not be
necessary, since we can always return EAGAIN, if the cache is stale,
right?

> 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).

This is true. But old software continues to exist and for INN there is
pretty much nothing else in this category available, I've been told by
several admins. Nobody really likes it, but it is used and improved
where necessary (epoll might be on the list already).

Regards

Ingo Oeser

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