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

[Don Lewis]

On  7 Oct, Konstantin Belousov wrote:
> 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.

I have no idea, but ghc would reliably fail to build on my Ryzen machine
up through r323398, and at r324367 it seems to reliably build.