NMI between switch_mm and switch_to

[Paul Mackerras]

Ben H. suggested there might be a problem if we get a PMU interrupt
and try to do a stack trace of userspace in the interval between when
we call switch_mm() from sched.c:context_switch() and when we call
switch_to().  If we get an NMI in that interval and do a stack trace
of userspace, we'll see the registers of the old task but when we peek
at user addresses we'll see the memory image for the new task, so the
stack trace we get will be completely bogus.

Is this in fact also a problem on x86, or is there some subtle reason
why it can't happen there?

Paul.
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