linux-kernel - Re: [Xen-devel] [PATCH 04/24] xen/arm: sync

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Date:	Fri, 27 Jul 2012 10:28:25 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"Tim (Xen.org)" <tim@....org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 04/24] xen/arm: sync_bitops

On Thu, 2012-07-26 at 17:37 +0100, Konrad Rzeszutek Wilk wrote:
> On Thu, Jul 26, 2012 at 04:33:46PM +0100, Stefano Stabellini wrote:
> > sync_bitops functions are equivalent to the SMP implementation of the
> > original functions, independently from CONFIG_SMP being defined.
> 
> So why can't the code be changed to use that? Is it that
> the _set_bit, _clear_bit, etc are not available with !CONFIG_SMP?

_set_bit etc are not SMP safe if !CONFIG_SMP. But under Xen you might be
communicating with a completely external entity who might be on another
CPU (e.g. two uniprocessor guests communicating via event channels and
grant tables). So we need a variant of the bit ops which are SMP safe
even on a UP kernel.

The users are common code and the sync_foo vs foo distinction matters on
some platforms (e.g. x86 where a UP kernel would omit the LOCK prefix
for the normal ones).

> 
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > ---
> >  arch/arm/include/asm/sync_bitops.h |   17 +++++++++++++++++
> >  1 files changed, 17 insertions(+), 0 deletions(-)
> >  create mode 100644 arch/arm/include/asm/sync_bitops.h
> > 
> > diff --git a/arch/arm/include/asm/sync_bitops.h b/arch/arm/include/asm/sync_bitops.h
> > new file mode 100644
> > index 0000000..d975092903
> > --- /dev/null
> > +++ b/arch/arm/include/asm/sync_bitops.h
> > @@ -0,0 +1,17 @@
> > +#ifndef __ASM_SYNC_BITOPS_H__
> > +#define __ASM_SYNC_BITOPS_H__
> > +
> > +#include <asm/bitops.h>
> > +#include <asm/system.h>
> > +
> > +#define sync_set_bit(nr, p)		_set_bit(nr, p)
> > +#define sync_clear_bit(nr, p)		_clear_bit(nr, p)
> > +#define sync_change_bit(nr, p)		_change_bit(nr, p)
> > +#define sync_test_and_set_bit(nr, p)	_test_and_set_bit(nr, p)
> > +#define sync_test_and_clear_bit(nr, p)	_test_and_clear_bit(nr, p)
> > +#define sync_test_and_change_bit(nr, p)	_test_and_change_bit(nr, p)
> > +#define sync_test_bit(nr, addr)		test_bit(nr, addr)
> > +#define sync_cmpxchg			cmpxchg
> > +
> > +
> > +#endif
> > -- 
> > 1.7.2.5
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@...ts.xen.org
> > http://lists.xen.org/xen-devel


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