[ltt-dev] [URCU PATCH] atomic: provide seq_cst semantics on powerpc

[Mathieu Desnoyers]

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> Using lwsync before a load conditional provides acq_rel semantics.  Other
> architectures provide sequential consistency, so replace the lwsync with
> sync (see also http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html).

Paul, I think Paolo is right here. And it seems to fit with the powerpc
barrier models we discussed at plumbers. Can you provide feedback on
this ?

Thanks,

Mathieu

> ---
>  urcu/uatomic/ppc.h |   18 ++++++------------
>  1 files changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/urcu/uatomic/ppc.h b/urcu/uatomic/ppc.h
> index 3eb3d63..8485f67 100644
> --- a/urcu/uatomic/ppc.h
> +++ b/urcu/uatomic/ppc.h
> @@ -27,12 +27,6 @@
>  extern "C" {
>  #endif 
>  
> -#ifdef __NO_LWSYNC__
> -#define LWSYNC_OPCODE	"sync\n"
> -#else
> -#define LWSYNC_OPCODE	"lwsync\n"
> -#endif
> -
>  #define ILLEGAL_INSTR	".long	0xd00d00"
>  
>  /*
> @@ -53,7 +47,7 @@ unsigned long _uatomic_exchange(void *addr, unsigned long val, int len)
>  		unsigned int result;
>  
>  		__asm__ __volatile__(
> -			LWSYNC_OPCODE
> +			"sync\n"		/* for sequential consistency */
>  		"1:\t"	"lwarx %0,0,%1\n"	/* load and reserve */
>  			"stwcx. %2,0,%1\n"	/* else store conditional */
>  			"bne- 1b\n"	 	/* retry if lost reservation */
> @@ -70,7 +64,7 @@ unsigned long _uatomic_exchange(void *addr, unsigned long val, int len)
>  		unsigned long result;
>  
>  		__asm__ __volatile__(
> -			LWSYNC_OPCODE
> +			"sync\n"		/* for sequential consistency */
>  		"1:\t"	"ldarx %0,0,%1\n"	/* load and reserve */
>  			"stdcx. %2,0,%1\n"	/* else store conditional */
>  			"bne- 1b\n"	 	/* retry if lost reservation */
> @@ -104,7 +98,7 @@ unsigned long _uatomic_cmpxchg(void *addr, unsigned long old,
>  		unsigned int old_val;
>  
>  		__asm__ __volatile__(
> -			LWSYNC_OPCODE
> +			"sync\n"		/* for sequential consistency */
>  		"1:\t"	"lwarx %0,0,%1\n"	/* load and reserve */
>  			"cmpw %0,%3\n"		/* if load is not equal to */
>  			"bne 2f\n"		/* old, fail */
> @@ -125,7 +119,7 @@ unsigned long _uatomic_cmpxchg(void *addr, unsigned long old,
>  		unsigned long old_val;
>  
>  		__asm__ __volatile__(
> -			LWSYNC_OPCODE
> +			"sync\n"		/* for sequential consistency */
>  		"1:\t"	"ldarx %0,0,%1\n"	/* load and reserve */
>  			"cmpd %0,%3\n"		/* if load is not equal to */
>  			"bne 2f\n"		/* old, fail */
> @@ -166,7 +160,7 @@ unsigned long _uatomic_add_return(void *addr, unsigned long val,
>  		unsigned int result;
>  
>  		__asm__ __volatile__(
> -			LWSYNC_OPCODE
> +			"sync\n"		/* for sequential consistency */
>  		"1:\t"	"lwarx %0,0,%1\n"	/* load and reserve */
>  			"add %0,%2,%0\n"	/* add val to value loaded */
>  			"stwcx. %0,0,%1\n"	/* store conditional */
> @@ -184,7 +178,7 @@ unsigned long _uatomic_add_return(void *addr, unsigned long val,
>  		unsigned long result;
>  
>  		__asm__ __volatile__(
> -			LWSYNC_OPCODE
> +			"sync\n"		/* for sequential consistency */
>  		"1:\t"	"ldarx %0,0,%1\n"	/* load and reserve */
>  			"add %0,%2,%0\n"	/* add val to value loaded */
>  			"stdcx. %0,0,%1\n"	/* store conditional */
> -- 
> 1.7.6
> 
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com