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

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Message-ID: <CA+55aFxuc3yD5pH2w5uEr_VDsEANvEiGCWA6tu1cSFrAiaU4Ow@mail.gmail.com>
Date:	Thu, 19 Sep 2013 13:11:32 -0500
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
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 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.

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

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

                   Linus
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