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Subject[cpuops cmpxchg double V2 0/4] this_cpu_cmpxchg_double support
This patch series introduces this_cpu_cmpxchg_double().

x86 cpus support cmpxchg16b and cmpxchg8b instuction which are capable of
switching two words instead of one during a cmpxchg.
Two words allow to swap more state in an atomic instruction.

this_cpu_cmpxchg_double() is used in the slub allocator to avoid
interrupt disable/enable in both alloc and free fastpath.
this_cpu_cmpxchg_double() works nicely with the per cpu data of the
allocator. Using the new operation significantly speeds up the fastpaths.

- Change parameter convention for this_cpu_cmpxchg_double. Specify both
percpu variables in same way as the two old and new values.
- Do not require a per cpu pointer but a variable to conform to the
convention used in other this_cpu_ops.

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