svn commit: r341682 - head/sys/sys

[Justin Hibbits]

On Mon, Dec 10, 2018, 17:57 Ian Lepore <ian at freebsd.org wrote:

> On Mon, 2018-12-10 at 14:15 -0800, John Baldwin wrote:
> > On 12/8/18 7:43 PM, Warner Losh wrote:
> > >
> > >
> > >
> > > On Sat, Dec 8, 2018, 8:36 PM Kevin Bowling <kevin.bowling at kev009.co
> > > m <mailto:kevin.bowling at kev009.com> wrote:
> > >
> > >     On Sat, Dec 8, 2018 at 12:09 AM Mateusz Guzik <mjguzik at gmail.co
> > > m <mailto:mjguzik at gmail.com>> wrote:
> > >
> > >     >
> > >     > Fully satisfying solution would be that all architectures get
> > > 64-bit
> > >     > ops, even if in the worst case they end up taking a lock.
> > > Then
> > >     > subsystems would not have to ifdef on anything. However,
> > > there
> > >     > was some opposition to this proposal and I don't think this
> > > is
> > >     > important enough to push.
> > >
> > >     Mateusz,
> > >
> > >     Who is opposing this particular polyfill solution?  Scott Long
> > > brought
> > >     up a situation in driver development where this would be useful
> > > as
> > >     well.  The polyfills lower the cognitive load and #ifdef soup
> > > which
> > >     are the right call here regardless of performance on toy ports.
> > >
> > >
> > > I don't recall seeing the opposition either. It would have to be a
> > > global lock for all 64bit atomics.... but I think it would only be
> > > 2 atomics on those architectures.
> > It would have to be a spin lock, so in the case of unrl you would be
> > trading
> > an operation on one of N regular mutexes for a single spin lock that
> > was
> > also contested by other things.  This would be pretty crappy.  For
> > drivers
> > that aren't actually used on platforms without 32-bit atomics we can
> > simply
> > not build them in sys/modules/Makefile or not put them in
> > GENERIC.  For
> > something in the core kernel like unrl I think we will have to do
> > what
> > Mateusz has done here.
> >
>
> On a single-core system all you need to implement 64-bit atomics in the
> kernel is to disable interrupts around using normal load/store
> operations on the values. Do we have any platforms that are SMP but
> don't have hardware primitives for 64-bit atomics?
>
> -- Ian
>

There were some dual processor G4 machines. I have one.  It doesn't have 64
bit atomics.

- Justin

>