linux-kernel - Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16

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Message-ID: <570A6078.2050002@linux.vnet.ibm.com>
Date:	Sun, 10 Apr 2016 22:17:28 +0800
From:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Boqun Feng <boqun.feng@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16


On 2016年04月08日 15:47, Peter Zijlstra wrote:
> On Fri, Apr 08, 2016 at 02:41:46PM +0800, Pan Xinhui wrote:
>> From: pan xinhui <xinhui.pan@...ux.vnet.ibm.com>
>>
>> Implement xchg{u8,u16}{local,relaxed}, and
>> cmpxchg{u8,u16}{,local,acquire,relaxed}.
>>
>> Atomic operation on 8-bit and 16-bit data type is supported from power7
> 
> And yes I see nothing P7 specific here, this implementation is for
> everything PPC64 afaict, no?
> 
Hello Peter,
	No, it's not for every ppc. So yes, I need add #ifdef here. Thanks for pointing it out.
We might need a new config option and let it depend on POWER7/POWER8_CPU or even POWER9...

> Also, note that you don't need explicit 8/16 bit atomics to implement
> these. Its fine to use 32bit atomics and only modify half the word.
> 
That is true. But I am a little worried about the performance. It will forbid any other tasks to touch the other half word during the load/reserve, right?
I am working on the qspinlock implementation on PPC.
Your and Waiman's patches are so nice. :)

> Also, you might want to invest in some CPP to reduce the endless
> repetition.
> 
Will do that. thanks for your tips.

thanks
xinhui
> Other than that, no objections :-)
>