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

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Message-ID: <20160105085117.GA11858@redhat.com>
Date:	Tue, 5 Jan 2016 10:51:17 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Boqun Feng <boqun.feng@...il.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

On Tue, Jan 05, 2016 at 09:36:55AM +0800, Boqun Feng wrote:
> 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

I think this is the part that was missed in review.

> > 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?

Yes.

> > -#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.

Absolutely. But smp_store_release (without __) will not.

Please note I did test this: for ppc code before and after
this patch generates exactly the same binary on SMP and 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

I think you missed the leading ___ :)

smp_store_release is external and it needs __smp_lwsync as
defined here.

I can duplicate some code and have smp_lwsync *not* call __smp_lwsync
but why do this? Still, if you prefer it this way,
please let me know.

> >  	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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