linux-kernel - Re: [PATCH 08/34] asm-generic: smp_store_mb should use smp

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Date:	Wed, 30 Dec 2015 22:30:38 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH 08/34] asm-generic: smp_store_mb should use smp_mb

On Wed, Dec 30, 2015 at 02:44:21PM +0100, Arnd Bergmann wrote:
> On Wednesday 30 December 2015 15:24:47 Michael S. Tsirkin wrote:
> > asm-generic variant of smp_store_mb() calls mb() which is stronger
> > than implied by both the name and the documentation.
> > 
> > smp_store_mb is only used by core kernel code at the moment, so
> > we know no one mis-uses it for an MMIO barrier.
> > Make it call smp_mb consistently before some arch-specific
> > code uses it as such by mistake.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  include/asm-generic/barrier.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> > index 538f8d1..987b2e0 100644
> > --- a/include/asm-generic/barrier.h
> > +++ b/include/asm-generic/barrier.h
> > @@ -93,7 +93,7 @@
> >  #endif	/* CONFIG_SMP */
> >  
> >  #ifndef smp_store_mb
> > -#define smp_store_mb(var, value)  do { WRITE_ONCE(var, value); mb(); } while (0)
> > +#define smp_store_mb(var, value)  do { WRITE_ONCE(var, value); smp_mb(); } while (0)
> >  #endif
> >  
> >  #ifndef smp_mb__before_atomic
> > 
> 
> The same patch is already in the tip tree scheduled for 4.5 as d5a73cadf3fd
> ("lcoking/barriers, arch: Use smp barriers in smp_store_release()").

Sorry which tree do you mean exactly?

> I think you can drop your version.
> 
> 	arnd

Will drop mine, thanks.
I kind of dislike that if I just drop it, some arches will temporarily
regress to a slower implementation.
I think I can just cherry-pick d5a73cadf3fd into my tree: git
normally figures such duplicates out nicely.
Does this sound good?

-- 
MST
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