illumos gate

To support CPR, cpu_pause can optionally invoke a custom callback. CPR uses this to save the processor state and power down the core. This callback is set via a global function pointer and it is up to the user of cpu-pausing to reset the function pointer to NULL to prevent future cpu-pausing from re-executing the callback. This is error prone and hard to reason about.

The callback setting can trivially be accomplished by passing the function pointer as an argument to pause_cpus. This clearly delimits the scope during which this callback is active (from the beginning of cpu-pausing to the time cpus are restarted), and it makes it impossible to not specify it and accidentally execute the last callback again.

The current callback setting can be retrieved from the global cpu_pause_info (cpu_pause_info.cp_func) in mdb.

1. cpu_lock must be held before the call to pause_cpus, and while start_cpus is called (not between the two, but only one CPU will be running and preemption is disabled)
2. cpu_lock serializes multiple threads attempting to pause cpus
3. since cpu_pause_info is only used during cpu pausing, cpu_pause_info is essentially procted by it