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

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Date:	Tue, 3 Feb 2009 14:05:43 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	corbet@....net, linux-kernel@...r.kernel.org, andi@...stfloor.org,
	oleg@...hat.com, viro@...IV.linux.org.uk, davidel@...ilserver.org,
	davem@...emloft.net, hch@....de, linux-api@...r.kernel.org,
	mpm@...enic.com, alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/4] Convert epoll to a bitlock

On Tue, 03 Feb 2009 22:55:26 +0100
Eric Dumazet <dada1@...mosbay.com> wrote:

> Andrew Morton a __crit :
> > On Mon,  2 Feb 2009 11:20:09 -0700
> > Jonathan Corbet <corbet@....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

ok..

> Probably an array of hashed spinlocks would be more than enough.
> 

yes, f_ep_lock is a teeny innermost lock.  Perhaps using
f->f_dentry->d_inode->i_lock would be a decent speed/space compromise.


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