linux-kernel - Re: [thisops uV2 09/10] x86: this_cpu_cmpxchg and this_cpu_cmpxchg

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Message-ID: <20101127152026.GA15758@Krystal>
Date:	Sat, 27 Nov 2010 10:20:26 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thisops uV2 09/10] x86: this_cpu_cmpxchg and
	this_cpu_cmpxchg_double operations

* Christoph Lameter (cl@...ux.com) wrote:
> Provide support as far as the hardware capabilities of the x86 cpus
> allow.
> 
> V1->V2:
> 	- Mark %rdx clobbering during cmpxchg16b
> 	- Provide emulation of cmpxchg16b for early AMD processors
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>

[...]
> +/*
> + * Something is screwed up with alternate instruction creation. This one
> + * fails with a mysterious asm error about a byte val > 255.

Hrm, "interesting" ;) Does each of these work if expressed directly in inline
assembly without the alternatives ? Can you give us more information about the
failure ?

> + */
> +#define percpu_cmpxchg16b(pcp, o1, o2, n1, n2)			\
> +({									\
> +	char __ret;							\
> +	typeof(o1) __o1 = o1;						\
> +	typeof(o1) __n1 = n1;						\
> +	typeof(o2) __o2 = o2;						\
> +	typeof(o2) __n2 = n2;						\
> +	typeof(o2) __dummy;						\
> +	VM_BUG_ON(((unsigned long)pcp) % 16);				\

Restricting typing on "pcp" at build time might be more appropriate. E.g.
creating a "struct doublecas" that would be forcefully aligned on 16 bytes. We
could check that with __builtin_types_compatible_p and generate a build failure
if necessary.

Thanks,

Mathieu

> +	alternative_io("call cmpxchg16b_local\n\t" P6_NOP4,		\
> +			"cmpxchg16b %%gs:(%%rsi)\n\tsetz %0\n\t",	\
> +			X86_FEATURE_CX16,				\
> +		    	ASM_OUTPUT2("=a"(__ret), "=d"(__dummy)),	\
> +		        "S" (pcp), "b"(__n1), "c"(__n2),		\
> +			 "a"(__o1), "d"(__o2));				\
> +	__ret;								\
> +})

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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