linux-kernel - Re: [PATCH v2 15/32] powerpc: define __smp

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Date:	Tue, 5 Jan 2016 09:36:55 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 15/32] powerpc: define __smp_xxx

Hi Michael,

On Thu, Dec 31, 2015 at 09:07:42PM +0200, Michael S. Tsirkin wrote:
> This defines __smp_xxx barriers for powerpc
> for use by virtualization.
> 
> smp_xxx barriers are removed as they are
> defined correctly by asm-generic/barriers.h
> 
> This reduces the amount of arch-specific boiler-plate code.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> Acked-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/powerpc/include/asm/barrier.h | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index 980ad0c..c0deafc 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -44,19 +44,11 @@
>  #define dma_rmb()	__lwsync()
>  #define dma_wmb()	__asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")
>  
> -#ifdef CONFIG_SMP
> -#define smp_lwsync()	__lwsync()
> +#define __smp_lwsync()	__lwsync()
>  

so __smp_lwsync() is always mapped to lwsync, right?

> -#define smp_mb()	mb()
> -#define smp_rmb()	__lwsync()
> -#define smp_wmb()	__asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")
> -#else
> -#define smp_lwsync()	barrier()
> -
> -#define smp_mb()	barrier()
> -#define smp_rmb()	barrier()
> -#define smp_wmb()	barrier()
> -#endif /* CONFIG_SMP */
> +#define __smp_mb()	mb()
> +#define __smp_rmb()	__lwsync()
> +#define __smp_wmb()	__asm__ __volatile__ (stringify_in_c(SMPWMB) : : :"memory")
>  
>  /*
>   * This is a barrier which prevents following instructions from being
> @@ -67,18 +59,18 @@
>  #define data_barrier(x)	\
>  	asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");
>  
> -#define smp_store_release(p, v)						\
> +#define __smp_store_release(p, v)						\
>  do {									\
>  	compiletime_assert_atomic_type(*p);				\
> -	smp_lwsync();							\
> +	__smp_lwsync();							\

, therefore this will emit an lwsync no matter SMP or UP.

Another thing is that smp_lwsync() may have a third user(other than
smp_load_acquire() and smp_store_release()):

http://article.gmane.org/gmane.linux.ports.ppc.embedded/89877

I'm OK to change my patch accordingly, but do we really want
smp_lwsync() get involved in this cleanup? If I understand you
correctly, this cleanup focuses on external API like smp_{r,w,}mb(),
while smp_lwsync() is internal to PPC.

Regards,
Boqun

>  	WRITE_ONCE(*p, v);						\
>  } while (0)
>  
> -#define smp_load_acquire(p)						\
> +#define __smp_load_acquire(p)						\
>  ({									\
>  	typeof(*p) ___p1 = READ_ONCE(*p);				\
>  	compiletime_assert_atomic_type(*p);				\
> -	smp_lwsync();							\
> +	__smp_lwsync();							\
>  	___p1;								\
>  })
>  
> -- 
> MST
> 
> --
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