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

From: Jan Beulich
Date: Wed Aug 27 2014 - 03:38:47 EST

>>> 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.


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