svn commit: r324313 - head/sys/amd64/amd64

[Konstantin Belousov]

On Sat, Oct 07, 2017 at 01:04:09PM -0700, Don Lewis wrote:
> On  5 Oct, Konstantin Belousov wrote:
> > Author: kib
> > Date: Thu Oct  5 12:50:03 2017
> > New Revision: 324313
> > URL: https://svnweb.freebsd.org/changeset/base/324313
> > 
> > Log:
> >   Avoid a race betweem freeing LDT and context switches.
> >   
> >   cpu_switch.S uses curproc->p_md.md_ldt value as the flag indicating
> >   presence of the process LDT.  The flag is checked and then ldt segment
> >   descriptor is copied into the CPU' GDT slot.
> >   
> >   Disallow context switches around clearing of the curproc LDT state by
> >   performing the cleanup in critical section.  Ensure that the md_ldt
> >   flag is cleared before md_ldt_sd descriptor content is destroyed by
> >   inserting fence between the operations.
> >   
> >   We depend on the x86 memory model strong ordering guarantees, in
> >   particular, that cpu_switch.S observes the writes to md_ldt and
> >   md_ldt_sd in the expected order.
> 
> I don't know which of this series of commits is responsible, but I think
> that it fixed the build of lang/ghc on Ryzen.
> 
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221029#c102

Does ghc use LDT on amd64 ?  This sounds unbelievable.