linux-kernel - Re: [PATCH v2 16/19] ARC: [plat-eznps] Use dedicated cpu

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Date:	Mon, 9 Nov 2015 11:05:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Noam Camus <noamc@...hip.com>
Cc:	linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
	talz@...hip.com, gilf@...hip.com, cmetcalf@...hip.com
Subject: Re: [PATCH v2 16/19] ARC: [plat-eznps] Use dedicated cpu_relax()

On Sat, Nov 07, 2015 at 12:52:34PM +0200, Noam Camus wrote:
> diff --git a/arch/arc/include/asm/processor.h b/arch/arc/include/asm/processor.h
> index 7266ede..50f9bae 100644
> --- a/arch/arc/include/asm/processor.h
> +++ b/arch/arc/include/asm/processor.h
> @@ -58,12 +58,21 @@ struct task_struct;
>   * get optimised away by gcc
>   */
>  #ifdef CONFIG_SMP
> +#ifndef CONFIG_EZNPS_MTM_EXT
>  #define cpu_relax()	__asm__ __volatile__ ("" : : : "memory")
>  #else
> +#define cpu_relax()     \
> +	__asm__ __volatile__ (".word %0" : : "i"(CTOP_INST_SCHD_RW) : "memory")
> +#endif
> +#else
>  #define cpu_relax()	do { } while (0)

I'm fairly sure this is incorrect. Even on UP we expect cpu_relax() to
be a compiler barrier.

>  #endif
>  
> +#ifndef CONFIG_EZNPS_MTM_EXT
>  #define cpu_relax_lowlatency() cpu_relax()
> +#else
> +#define cpu_relax_lowlatency() barrier()
> +#endif

At which point you can unconditionally use that definition.

>  
>  #define copy_segments(tsk, mm)      do { } while (0)
>  #define release_segments(mm)        do { } while (0)
> -- 
> 1.7.1
> 
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