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

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Message-ID: <alpine.DEB.2.02.1208011707280.4645@kaball.uk.xensource.com>
Date:	Wed, 1 Aug 2012 17:07:42 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	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 Wed, 1 Aug 2012, Konrad Rzeszutek Wilk wrote:
> On Fri, Jul 27, 2012 at 10:28:25AM +0100, Ian Campbell wrote:
> > 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).
> 
> OK, that makes sense. Stefano can you include that comment in the git
> commit description and in the sync_bitops.h file please?

Yep, I'll do that.

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