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kernelkernel-modulemach

Mac OS X Kernel Extension CPU Selection

I was reading the source code for the model-specific register (MSR) driver kernel extension that ships with the Intel Performance Counter Monitor (http://software.intel.com/en-us/articles/intel-performance-counter-monitor-a-better-way-to-measure-cpu-utilization). Since independent copies of the MSRs/performance counters are stored on different CPUs, it is necessary to specify which CPU to read from. This is done by calling the mp_rendezvous_no_intrs function. 

mp_rendezvous_no_intrs(cpuReadMSR, (void*)idatas);

This causes each processor the check if it is the correct processor number, and if so read the data from the MSR: 

void cpuReadMSR(void* pIData){
    pcm_msr_data_t* data = (pcm_msr_data_t*)pIData;
    volatile uint cpu = cpu_number();
    if(data->cpu_num == cpu)
    {
        data->value = RDMSR(data->msr_num);
    }
}

My question is: is turning off interrupts (via however mp_rendezvous_no_intrs does it) enough to cause the thread running the cpuReadMSR function to stay on the same CPU the whole time? If it is not, I worry about the following failure scenario:

  1. The cpu reads its numerical id with cpu_number(), decides that it is the correct CPU, and starts to read the MSR.
  2. The thread is preempted by the scheduler and moved to a different CPU.
  3. The MSR is read, but now is read from a different CPU, thus giving the wrong value.
Solution

  • Disabling interrupts disables ALL interrupts, not just some of them. This includes the timer interrupt, which is what normally allows a running thread to be preempted.

    While interrupts are disabled, nothing (short of something crazy like a CPU exception) can interrupt your code from running, start to finish, on a single CPU.