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

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Date:	Thu, 19 Sep 2013 22:59:10 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Will Deacon <will.deacon@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates

On 09/19/2013 02:11 PM, 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.
> 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

I agreed that 32-bit machines are not likely to be big enough to get 
benefit from this feature. However, I do see a minor problem with the 
current code. If a user tries to turn on CMPXCHG_LOCKREF on a 32-bit 
build, he/she will get a compilation error because a 64-bit data type is 
not supported in 32-bit mode's cmpxchg(). Because of this, my original 
patch also uses cmpxchg64() which is equivalent to cmpxchg() in 64-bit 
machine for 64-bit data type.

I would suggest either change to use cmpxchg64() or add the dependence 
line "depends on 64BIT" to CMPXCHG_LOCKREF.

-Longman
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