stop_machine: ipipe: ensure atomic stop-context operations
stop_machine() guarantees that all online CPUs are spinning non-preemptible in a known code location before a subset of them may safely run a stop-context function. This service is typically useful for live patching the kernel code, or changing global memory mappings, so that no activity could run in parallel until the system has returned to a stable state after all stop-context operations have completed. When interrupt pipelining is enabled, we have to provide the same guarantee by restoring hard interrupt disabling where virtualizing the interrupt disable flag would defeat it.
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