netdev - Re: [patch sungem] improved locking

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Date:	Wed, 29 Nov 2006 11:16:38 +0100
From:	"Eric Lemoine" <eric.lemoine@...il.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	benh@...nel.crashing.org, netdev@...r.kernel.org
Subject: Re: [patch sungem] improved locking

On 11/29/06, David Miller <davem@...emloft.net> wrote:
> From: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Date: Wed, 29 Nov 2006 09:57:24 +1100
>
> >
> > > This looks mostly fine.
> > >
> > > I was thinking about the lockless stuff, and I wonder if there
> > > is a clever way you can get it back down to one PIO on the
> > > GREG_STAT register.
> > >
> > > I think you'd need to have the ->poll() clear gp->status, then
> > > do a smp_wb(), right before it re-enables interrupts.
> > >
> > > Then in the interrupt handler, you need to find a way to safely
> > > OR-in any unset bits in gp->status in a race-free manner.
> >
> > Having it atomic might work at a slightly smaller cost than a lock,
> > though atomics don't have strong ordering requirements so you'd still
> > have to be a bit careful.
>
> At least in theory the atomic + any necessary memory barriers
> would be cheaper than the extra PIO read we need otherwise.

Just a remark: the lockless impl doesn't add a PIO read, it adds a PIO
write (to the IACK reg). FYI, I'm currently checking whether I can get
rid of this extra PIO write, based on David's suggestion...


-- 
Eric
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