Subject: Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16

From: Peter Zijlstra <[email-masked]>
Date: Tue, 12 Apr 2016 16:30:23 +0200

On Sun, Apr 10, 2016 at 10:17:28PM +0800, Pan Xinhui wrote:
> 
> 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 <[email-masked]>
> >>
> >> 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...

Right, I'm not sure if PPC has alternatives, but you could of course
runtime patch the code from emulated with 32bit ll/sc to native 8/16bit
ll/sc if present on the current CPU if you have infrastructure for these
things.

> > 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?

Well, not forbid, it would just make the LL/SC fail and try again. Other
archs already implement them this way. See commit 3226aad81aa6 ("sh:
support 1 and 2 byte xchg") for example.

> I am working on the qspinlock implementation on PPC.
> Your and Waiman's patches are so nice. :)

Thanks!, last time I looked at PPC spinlocks they could not use things
like ticket locks because PPC might be a guest and fairness blows etc..

You're making the qspinlock-paravirt thing work on PPC, or doing
qspinlock only for bare-metal PPC?