svn commit: r324313 - head/sys/amd64/amd64
Don Lewis
truckman at FreeBSD.org
Sat Oct 7 20:04:20 UTC 2017
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
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