linux-kernel - RE: sync_set_bit() vs set_bit() -- what's the difference?

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Message-ID: <EE124450C0AAF944A40DD71E61F878C9963AA0@SINEX14MBXC419.southpacific.corp.microsoft.com>
Date:	Wed, 27 Aug 2014 07:50:52 +0000
From:	Dexuan Cui <decui@...rosoft.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	"jeremy@...p.org" <jeremy@...p.org>,
	KY Srinivasan <kys@...rosoft.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: sync_set_bit() vs set_bit()  -- what's the difference?

> -----Original Message-----
> From: Jan Beulich
> Sent: Wednesday, August 27, 2014 15:39 PM
> >>> On 27.08.14 at 09:30, <decui@...rosoft.com> wrote:
> > I'm curious about the difference. :-)
> >
> > sync_set_bit() is only used in drivers/hv/ and drivers/xen/ while set_bit()
> > is used in all other places. What makes hv/xen special?
> 
> I guess this would really want to be used by anything communicating
> with a hypervisor or a remote driver: set_bit() gets its LOCK prefix
> discarded when the local kernel determines it runs on a single CPU
> only. Obviously having knowledge of the CPU count inside a VM does
> not imply anything about the number of CPUs available to the host,
> i.e. stripping LOCK prefixes in that case would be unsafe.
> 
> Jan

Thank you, Juergen and Jan for your  quick answers!

I didn't realize LOCK_PREFIX is "" for UP. :-)

Thanks,
-- Dexuan
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