svn commit: r341682 - head/sys/sys

[Ian Lepore]

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