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

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Message-ID: <032994b8585e483c86e71402ecd5a163@BY2PR0301MB0711.namprd03.prod.outlook.com>
Date:	Wed, 27 Aug 2014 13:56:27 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Jan Beulich <JBeulich@...e.com>, Dexuan Cui <decui@...rosoft.com>
CC:	"jeremy@...p.org" <jeremy@...p.org>,
	"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 [mailto:JBeulich@...e.com]
> Sent: Wednesday, August 27, 2014 12:39 AM
> To: Dexuan Cui
> Cc: jeremy@...p.org; KY Srinivasan; chrisw@...s-sol.org; linux-
> kernel@...r.kernel.org
> Subject: Re: sync_set_bit() vs set_bit() -- what's the difference?
> 
> >>> 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.

That is exactly the case for Hyper-V (and Xen).

K. Y

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