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

[Matt Mackall <mpm-VDJrAJ4Gl5ZBDgjK7y7TUQ@public.gmane.org>]

On Tue, 2009-02-03 at 14:05 -0800, Andrew Morton wrote:
> On Tue, 03 Feb 2009 22:55:26 +0100
> Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org> wrote:
> 
> > Andrew Morton a __crit :
> > > On Mon,  2 Feb 2009 11:20:09 -0700
> > > Jonathan Corbet <corbet-T1hC0tSOHrs@public.gmane.org> 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.

That seems eminently reasonable.

But that re-opens the question of what to do about poor Jon's quest.

I got confused halfway through as he went from using a global fasync
spinlock to a non-locked but atomic flag bit. Not sure why using a
per-file or per-inode lock doesn't work for the fasync code.

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