linux-kernel - Re: [PATCH] arch,locking: Ciao arch_mutex_cpu

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Date:	Mon, 23 Jun 2014 08:54:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	linux-kernel@...r.kernel.org, vgupta@...opsys.com
Subject: Re: [PATCH] arch,locking: Ciao arch_mutex_cpu_relax()

On Fri, Jun 20, 2014 at 11:21:13AM -0700, Davidlohr Bueso wrote:
> diff --git a/arch/arc/include/asm/processor.h b/arch/arc/include/asm/processor.h
> index d99f9b3..8e1bf6b 100644
> --- a/arch/arc/include/asm/processor.h
> +++ b/arch/arc/include/asm/processor.h
> @@ -62,6 +62,8 @@ unsigned long thread_saved_pc(struct task_struct *t);
>  #define cpu_relax()	do { } while (0)
>  #endif
>  
> +#define arch_cpu_relax() cpu_relax()
> +
>  #define copy_segments(tsk, mm)      do { } while (0)
>  #define release_segments(mm)        do { } while (0)

I'm not at all sure that cpu_relax() definition ARC has is valid. We
rely on cpu_relax() being at least a barrier() all over the place, and
it doesn't need to be SMP only. You can have a UP wait loop waiting for
an interrupt for example.

Vineet?
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