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

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Date:	Wed, 27 Aug 2014 09:58:50 +0200
From:	Jürgen Groß <jgross@...e.com>
To:	Dexuan Cui <decui@...rosoft.com>, 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?

On 08/27/2014 09:50 AM, Dexuan Cui wrote:
>> -----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. :-)

Even worse: it is patched away dynamically when you disable all but one
processor and activated again when a second processor is becoming
active.


Juergen

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