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

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Date:	Wed, 27 Aug 2014 08:38:35 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Dexuan Cui" <decui@...rosoft.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?

>>> 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

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