descriptor for the LDT) is populated. md_ldt is used by context-switch
code as indicator that LDT segment register shall be loaded with
GUSERLDT segment instead of 0, so context switch at the wrong time may
cause attempt to load non-populated descriptor.
Use store with the barrier to prevent other CPUs from seeing updated
md_ldt but not seeing updated md_ldt_sd. Multithreaded process may
context-switch to another thread of the process on another CPU and read
md_ldt.
MFC after: 1 week
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