On Wed, Sep 07, 2016 at 08:13:31AM +0100, Alex Bennée wrote: > > David Gibson <da...@gibson.dropbear.id.au> writes: > > > On Wed, Sep 07, 2016 at 10:17:42AM +0530, Nikunj A Dadhania wrote: > >> David Gibson <da...@gibson.dropbear.id.au> writes: > >> > >> > [ Unknown signature status ] > >> > On Fri, Sep 02, 2016 at 12:02:55PM +0530, Nikunj A Dadhania wrote: > >> >> Signed-off-by: Nikunj A Dadhania <nik...@linux.vnet.ibm.com> > >> > > >> > This really needs a comment indicating that this implementation isn't > >> > strictly correct (although probably good enough in practice). > >> > >> Sure. And it also does not help if someone uses any store other than > >> store conditional, that isn't taken care. > >> > >> Assumption here is the locking primitives use load with reservation and > >> store conditional. And no other ld/st variant touch this memory. > > > > So, a) I don't think this really relies on that: an ordinary store > > (assuming it changes the value) will still get picked up the cmpxchg. > > Well.. at least after a suitable memory barrier. Matching memory > > models between emulated and host cpus is a whole other kettle of fish. > > Have you seen Pranith's memory barrier patches?
I have not.
>
> >
> > I think this does matter, IIRC a kernel spin unlock on ppc is a
> > barrier + plain store, no load locked or store conditional.
> >
> >> > Specifically a racing store which happens to store the same value
> >> > which was already in memory should clobber the reservation, but won't
> >> > with this implementation.
> >> >
> >> > I had a long discussion at KVM Forum with Emilio Costa about this, in
> >> > which I discovered just how hard it is to strictly implement
> >> > store-conditional semantics in terms of anything else. So, this is
> >> > probably a reasonable substitute, but we should note the fact that
> >> > it's not 100%.
> >>
> >> I will update the commit log.
> >>
> >> Regards,
> >> Nikunj
> >>
>
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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