[PATCH 1/2] x86/dumpstack: Optimize save_stack_trace

[Byungchul Park]

I suggested this patch on https://lkml.org/lkml/2016/6/20/22. However,
I want to proceed saperately since it's somewhat independent from each
other. Frankly speaking, I want this patchset to be accepted at first so
that the crossfeature can use this optimized save_stack_trace_norm()
which makes crossrelease work smoothly.

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