svn commit: r341682 - head/sys/sys

[Konstantin Belousov]

On Mon, Dec 10, 2018 at 02:15:20PM -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.com <mailto:kevin.bowling at kev009.com> wrote:
> > 
> >     On Sat, Dec 8, 2018 at 12:09 AM Mateusz Guzik <mjguzik at gmail.com <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.

It is worse. All atomics that acess the same location must use the same
lock. Otherwise, you could observe torn writes and out of thin air
values. Since you cannot know in advance which locations are acceses
by the locked variant, all freebsd atomics ops have to be switched to
locked variant on the architecture.