MacOS X hibernate path and preemption

Below is a MacOS X 10.9 callstack while processing a request to hibernate

mach_kernel`IOPMrootDomain::pmStatsRecordEvent() + 227 at IOPMrootDomain.cpp:6969

mach_kernel`hibernate_machine_init + 3160 at IOHibernateIO.cpp:3112

mach_kernel`acpi_sleep_kernel() + 503 at acpi.c:320

AppleACPIPlatform`AppleACPIPlatformExpert::sleepPlatform() + 443

AppleACPIPlatform`AppleACPICPU::haltCPU() + 117

mach_kernel`IOCPUSleepKernel() + 764 at IOCPU.cpp:403

mach_kernel`IOPMrootDomain::powerChangeDone() + 531 at IOPMrootDomain.cpp:2256

mach_kernel`IOService::all_done() + 1221 at IOServicePM.cpp:4269

mach_kernel`IOService::servicePMRequest(IOPMRequest*, IOPMWorkQueue*) [inlined] IOService::OurChangeFinish() + 8 at IOServicePM.cpp:4736

mach_kernel`IOService::servicePMRequest() + 2773 at IOServicePM.cpp:7337

mach_kernel`IOPMWorkQueue::checkRequestQueue() + 52 at IOServicePM.cpp:8236

mach_kernel`IOPMWorkQueue::checkForWork() + 127 at IOServicePM.cpp:8296

mach_kernel`IOWorkLoop::runEventSources() + 258 at IOWorkLoop.cpp:367

mach_kernel`IOWorkLoop::threadMain() + 195 at IOWorkLoop.cpp:395

The call to IOPMrootDomain::pmStatsRecordEvent is done with preemption disabled, i.e. the threads scheduling is not allowed, but IOPMrootDomain::pmStatsRecordEvent calls IORegistryEntry::setProperty that acquires a mutex and mutex acquisition can block a thread calling the scheduler in case of contention for mutex. So, is this a bug in the Apple code? I do not know but there is a workaround in the Apple code to not panic a debug kernel build when calling IORegistryEntry::setProperty with preemption disabled - a check cmpl $0,%gs:CPU_HIBERNATE bypasses the whole CHECK_PREEMPTION_LEVEL macro if a CPU is in hibernating mode. It makes sense if there are no other active CPUs in the system.

An excerpt from i386_locks.s

/*

 * If one or more simplelocks are currently held by a thread,

 * an attempt to acquire a mutex will cause this check to fail

 * (since a mutex lock may context switch, holding a simplelock

 * is not a good thing).

 */

#if MACH_RT

#define CHECK_PREEMPTION_LEVEL() \

cmpl $0,%gs:CPU_HIBERNATE ; \

jne 1f ; \

cmpl $0,%gs:CPU_PREEMPTION_LEVEL ; \

je 1f ; \

ALIGN_STACK() ; \

movl %gs:CPU_PREEMPTION_LEVEL, %eax ; \

LOAD_ARG1(%eax) ; \

LOAD_STRING_ARG0(2f) ; \

CALL_PANIC() ; \

hlt ; \

.data ; \

2: String "preemption_level(%d) != 0!" ; \

.text ; \

1:

#else /* MACH_RT */

#define CHECK_PREEMPTION_LEVEL()

#endif /* MACH_RT */