linux-kernel - Re: [PATCH v2 22/32] s390: define __smp

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Message-ID: <20160104221323-mutt-send-email-mst@redhat.com>
Date:	Mon, 4 Jan 2016 22:18:58 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.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,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v2 22/32] s390: define __smp_xxx

On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote:
> > This defines __smp_xxx barriers for s390,
> > for use by virtualization.
> > 
> > Some smp_xxx barriers are removed as they are
> > defined correctly by asm-generic/barriers.h
> > 
> > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers
> > unconditionally on this architecture.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > Acked-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  arch/s390/include/asm/barrier.h | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h
> > index c358c31..fbd25b2 100644
> > --- a/arch/s390/include/asm/barrier.h
> > +++ b/arch/s390/include/asm/barrier.h
> > @@ -26,18 +26,21 @@
> >  #define wmb()				barrier()
> >  #define dma_rmb()			mb()
> >  #define dma_wmb()			mb()
> > -#define smp_mb()			mb()
> > -#define smp_rmb()			rmb()
> > -#define smp_wmb()			wmb()
> > -
> > -#define smp_store_release(p, v)						\
> > +#define __smp_mb()			mb()
> > +#define __smp_rmb()			rmb()
> > +#define __smp_wmb()			wmb()
> > +#define smp_mb()			__smp_mb()
> > +#define smp_rmb()			__smp_rmb()
> > +#define smp_wmb()			__smp_wmb()
> 
> Why define the smp_*mb() primitives here? Would not the inclusion of
> asm-generic/barrier.h do this?

No because the generic one is a nop on !SMP, this one isn't.

Pls note this patch is just reordering code without making
functional changes.
And at the moment, on s390 smp_xxx barriers are always non empty.

Some of this could be sub-optimal, but
since on s390 Linux always runs on a hypervisor,
I am not sure it's safe to use the generic version -
in other words, it just might be that for s390 smp_ and virt_
barriers must be equivalent.

If in fact this turns out to be wrong, I can pick up
a patch to change this, but I'd rather make this
a patch on top so that my patches are testable
just by compiling and comparing the binary.

-- 
MST
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