Re: [PATCH 2/4] Convert epoll to a bitlock

Davide Libenzi
2009-02-03T23:08:33Z

[-- Attachment #1: Type: TEXT/PLAIN, Size: 1243 bytes --]

On Tue, 3 Feb 2009, Eric Dumazet wrote:

> Andrew Morton a écrit :
> > On Mon, 2 Feb 2009 11:20:09 -0700
> > Jonathan Corbet <corbet@lwn.net> wrote:
> >
> >> Matt Mackall suggested converting epoll's ep_lock to a bitlock as a way of
> >> saving space in struct file. This patch makes that change.
> >
> > hrm. bit_spin_lock() makes people upset (large penguiny people). iirc
> > it doesn't have all the correct/well-understood memory/compiler
> > ordering semantics which spinlocks have. And lockdep doesn't know about
> > it.
> >
>
> In a previous attempt (2005), I suggested using a single global lock.
>
> http://search.luky.org/linux-kernel.2005/msg50862.html
>
> Probably an array of hashed spinlocks would be more than enough.

That could be done, although I'm not sure it's worth going that way to
save 4 bytes. The effective saving rate is not even 4/sizeof(struct file)
since struct file never comes alone, and when you allocate a struct file
you always carry more allocations behind (at least for the cases where you
tend to have a lot of them around, so size would matter).
The add/remove path in epoll is not a super-hot one, so it could be done.
I dunno how this change matter with the patchset though.

- Davide