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From: Don Lewis <truckman@FreeBSD.org>
Subject: Re: svn commit: r324313 - head/sys/amd64/amd64
To: kib@FreeBSD.org
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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