context switch

["Ky Srinivasan" <ksrinivasan@novell.com>]

In debugging the sles9 port on 64 bit MP machines, I am seeing a problem
where the hypervisor takes a fault in loading fs in the context switch
code (load_segments()). The selector is one of the TLS selectors. It
appears that the cpu in question has updated this selector with a value
of 0 just prior to the problem I am seeing. Looking at the Linux context
switch code, we first update the TLS selector values of the incoming
context  before we load the segment registers. So, if we preempt the CPU
after it has modified the gdt table but before it loads up the segment
registers, we could get into a situation where when the hypervisor
resumes the preempted domain on this cpu, we could fault on the segment
register load. I am  curious to understand why this is not an issue. How
are such windows closed. 

Regards,

K. Y