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

[Jürgen Groß]

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@xxxxxxxxxxxxx> 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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