linux-kernel - Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates

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Date:	Fri, 20 Sep 2013 11:08:07 +0100
From:	Will Deacon <will.deacon@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates

On Thu, Sep 19, 2013 at 07:11:32PM +0100, Linus Torvalds wrote:
> On Thu, Sep 19, 2013 at 1:06 PM, Will Deacon <will.deacon@....com> wrote:
> > The cmpxchg() function tends not to support 64-bit arguments on 32-bit
> > architectures. This could be either due to use of unsigned long arguments
> > (like on ARM) or lack of instruction support (cmpxchgq on x86). However,
> > these architectures may implement a specific cmpxchg64() function to
> > provide 64-bit cmpxchg support instead
> 
> I'm certainly ok with this, but I wonder how much point there is to
> use the cmpxchg alternatives for 32-bit architectures at all...
> 
> From a performance standpoint, lockref really is expected to mainly
> help with big machines. Only insane people would do big machines with
> 32-bit kernels these days.

Our definitions of "big" machines probably differ significantly, but it
would be interesting to see if this *does* make a difference on some of the
multi-cluster ARMv7 hardware. Unfortunately, my development boards are all
I/O bound, so I'll need to leave a strategically placed crate of
non-poisoned beer next to the server guys' office...

> Of course, it may be that cmpxchg is actually faster on some
> architectures, but at least on x86-32, cmpxchg8b is traditionally
> quite slow.

On ARMv7, our double-word exclusives shouldn't be slower than the word
exclusives (hell, everything apart from the machine registers will be >=
64-bit).

> In other words, I'd actually like to see some numbers if there are
> loads where this actually helps and matters...

That's fair enough; I just saw the new lockref stuff, thought "that's a cool
hack" then looked at playing with it on ARM. I'll go see what this AIM7
thing is all about...

Cheers,

Will
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