zircon/kernel/tests/preempt_disable_tests.cc - fuchsia - Git at Google

 // Copyright 2018 The Fuchsia Authors
 //
 // Use of this source code is governed by a MIT-style
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT

 #include <lib/unittest/unittest.h>
 #include <platform.h>

 #include <kernel/auto_preempt_disabler.h>
 #include <kernel/event.h>
 #include <kernel/interrupt.h>
 #include <kernel/scheduler.h>
 #include <kernel/thread_lock.h>
 #include <kernel/timer.h>
 #include <ktl/atomic.h>

 // Test that preempt_disable is set for timer callbacks and that, in this
 // context, preempt_pending will get set by some functions.
 static void timer_callback_func(Timer* timer, zx_time_t now, void* arg) {
   Event* event = (Event*)arg;

   // Timer callbacks should be called in interrupt context with
   // preempt_disable set.
   ASSERT(arch_ints_disabled());
   ASSERT(arch_blocking_disallowed());
   PreemptionState& preemption_state = Thread::Current::preemption_state();
   ASSERT(preemption_state.PreemptDisableCount() > 0);

   // Save and restore the value of preempt_pending so that we can test
   // other functions' behavior with preempt_pending==false.  It is
   // possible that preempt_pending is true now: it might have been set by
   // another timer callback.
   bool old_preempt_pending = preemption_state.preempt_pending();

   // Test that Scheduler::Reschedule() sets the preempt_pending flag when
   // preempt_disable is set.
   preemption_state.preempt_pending() = false;
   {
     Guard<SpinLock, IrqSave> guard{ThreadLock::Get()};
     Scheduler::Reschedule();
   }
   ASSERT(preemption_state.preempt_pending());

   // Test that preemption_state.PreemptSetPending() sets the preempt_pending
   // flag.
   preemption_state.preempt_pending() = false;
   preemption_state.PreemptSetPending();
   ASSERT(preemption_state.preempt_pending());

   // Restore value.
   preemption_state.preempt_pending() = old_preempt_pending;

   event->Signal();
 }

 // Schedule a timer callback and wait for it to complete.  Most of the
 // testing is done in the timer callback.
 static bool test_in_timer_callback() {
   BEGIN_TEST;

   Event event;

   Timer timer;
   timer.Set(Deadline::no_slack(0), timer_callback_func, &event);

   ASSERT_EQ(event.Wait(), ZX_OK);

   END_TEST;
 }

 // Test incrementing and decrementing the preempt_disable and
 // resched_disable counts.
 static bool test_inc_dec_disable_counts() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   // Test initial conditions.
   ASSERT_EQ(preemption_state.PreemptDisableCount(), 0u);
   ASSERT_EQ(preemption_state.EagerReschedDisableCount(), 0u);
   // While preemption is allowed, a preemption should not be pending.
   ASSERT_EQ(preemption_state.preempt_pending(), false);

   // Test incrementing and decrementing of preempt_disable.
   preemption_state.PreemptDisable();
   EXPECT_EQ(preemption_state.PreemptDisableCount(), 1u);
   preemption_state.PreemptReenable();
   EXPECT_EQ(preemption_state.PreemptDisableCount(), 0u);

   // Test incrementing and decrementing of resched_disable.
   preemption_state.EagerReschedDisable();
   EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 1u);
   preemption_state.EagerReschedReenable();
   EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 0u);

   // Test nesting: multiple increments and decrements.
   preemption_state.PreemptDisable();
   preemption_state.PreemptDisable();
   EXPECT_EQ(preemption_state.PreemptDisableCount(), 2u);
   preemption_state.PreemptReenable();
   preemption_state.PreemptReenable();
   EXPECT_EQ(preemption_state.PreemptDisableCount(), 0u);

   // Test nesting: multiple increments and decrements.
   preemption_state.EagerReschedDisable();
   preemption_state.EagerReschedDisable();
   EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 2u);
   preemption_state.EagerReschedReenable();
   preemption_state.EagerReschedReenable();
   EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 0u);

   END_TEST;
 }

 static bool test_decrement_clears_preempt_pending() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   // Test that preemption_state.PreemptReenable() clears preempt_pending.
   preemption_state.PreemptDisable();
   Thread::Current::Reschedule();
   // It should not be possible for an interrupt handler to block or
   // otherwise cause a reschedule before our preemption_state.PreemptReenable().
   EXPECT_EQ(preemption_state.preempt_pending(), true);
   preemption_state.PreemptReenable();
   EXPECT_EQ(preemption_state.preempt_pending(), false);

   // Test that preemption_state.ReschedReenable() clears preempt_pending.
   preemption_state.EagerReschedDisable();
   interrupt_saved_state_t int_state = arch_interrupt_save();
   Thread::Current::Reschedule();
   // Read preempt_pending with interrupts disabled because otherwise an
   // interrupt handler could set it to false.
   EXPECT_EQ(preemption_state.preempt_pending(), true);
   arch_interrupt_restore(int_state);
   preemption_state.EagerReschedReenable();
   EXPECT_EQ(preemption_state.preempt_pending(), false);

   END_TEST;
 }

 static bool test_blocking_clears_preempt_pending() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   // It is OK to block while preemption is disabled.  In this case,
   // blocking should clear preempt_pending.
   preemption_state.PreemptDisable();
   Thread::Current::Reschedule();
   EXPECT_EQ(preemption_state.preempt_pending(), true);
   interrupt_saved_state_t int_state = arch_interrupt_save();
   Thread::Current::SleepRelative(ZX_MSEC(10));
   // Read preempt_pending with interrupts disabled because otherwise an
   // interrupt handler could set it to true.
   EXPECT_EQ(preemption_state.preempt_pending(), false);
   arch_interrupt_restore(int_state);
   preemption_state.PreemptReenable();

   // It is OK to block while rescheduling is disabled.  In this case,
   // blocking should clear preempt_pending.
   preemption_state.EagerReschedDisable();
   Thread::Current::Reschedule();
   Thread::Current::SleepRelative(ZX_MSEC(10));
   EXPECT_EQ(preemption_state.preempt_pending(), false);
   preemption_state.EagerReschedReenable();

   END_TEST;
 }

 // Test that preempt_pending is preserved across an interrupt handler when
 // resched_disable is set and when the interrupt handler does not cause a
 // preemption.  This tests the int_handler_start()/finish() routines rather
 // than the full interrupt handler.
 static bool test_interrupt_preserves_preempt_pending() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   preemption_state.EagerReschedDisable();
   // Do this with interrupts disabled so that a real interrupt does not
   // clear preempt_pending.
   interrupt_saved_state_t int_state = arch_interrupt_save();
   Thread::Current::Reschedule();

   // Simulate an interrupt handler invocation.
   int_handler_saved_state_t state;
   int_handler_start(&state);
   EXPECT_EQ(preemption_state.PreemptDisableCount(), 1u);
   bool do_preempt = int_handler_finish(&state);

   EXPECT_EQ(do_preempt, false);
   EXPECT_EQ(preemption_state.preempt_pending(), true);
   arch_interrupt_restore(int_state);
   preemption_state.EagerReschedReenable();
   EXPECT_EQ(preemption_state.preempt_pending(), false);

   END_TEST;
 }

 // Test that resched_disable does not prevent preemption by an interrupt
 // handler.
 static bool test_interrupt_clears_preempt_pending() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   preemption_state.EagerReschedDisable();
   Thread::Current::Reschedule();
   // Spin until we detect that we have been preempted.  preempt_pending
   // should eventually get set to false because the scheduler should
   // preempt this thread.
   while (preemption_state.preempt_pending()) {
   }
   preemption_state.EagerReschedReenable();

   END_TEST;
 }

 // Timer callback used for testing.
 static void timer_set_preempt_pending(Timer* timer, zx_time_t now, void* arg) {
   auto* timer_ran = reinterpret_cast<ktl::atomic<bool>*>(arg);

   Thread::Current::preemption_state().PreemptSetPending();
   timer_ran->store(true);
 }

 static bool test_interrupt_with_preempt_disable() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   // Test that interrupt handlers honor preempt_disable.
   //
   // We test that by setting a timer callback that will set
   // preempt_pending from inside an interrupt handler.  preempt_pending
   // should remain set after the interrupt handler returns.
   //
   // This assumes that timer_set() will run the callback on the same CPU
   // that we invoked it from.  This also assumes that we don't
   // accidentally call any blocking operations that cause our thread to
   // be rescheduled to another CPU.
   preemption_state.PreemptDisable();
   ktl::atomic<bool> timer_ran(false);
   Timer timer;
   const Deadline deadline = Deadline::after(ZX_USEC(100));
   timer.Set(deadline, timer_set_preempt_pending, reinterpret_cast<void*>(&timer_ran));
   // Spin until timer_ran is set by the interrupt handler.
   while (!timer_ran.load()) {
   }
   EXPECT_EQ(preemption_state.preempt_pending(), true);
   preemption_state.PreemptReenable();

   END_TEST;
 }

 static bool test_interrupt_with_resched_disable() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   // Test that resched_disable does not defer preemptions by interrupt
   // handlers.
   //
   // This assumes that timer_set() will run the callback on the same CPU
   // that we invoked it from.
   preemption_state.EagerReschedDisable();
   ktl::atomic<bool> timer_ran(false);
   Timer timer;
   const Deadline deadline = Deadline::after(ZX_USEC(100));
   timer.Set(deadline, timer_set_preempt_pending, reinterpret_cast<void*>(&timer_ran));
   // Spin until timer_ran is set by the interrupt handler.
   while (!timer_ran.load()) {
   }
   // preempt_pending should be reset to false either on returning from
   // our timer interrupt, or by some other preemption.
   EXPECT_EQ(preemption_state.preempt_pending(), false);
   preemption_state.EagerReschedReenable();

   END_TEST;
 }

 static bool test_auto_preempt_disabler() {
   BEGIN_TEST;

   PreemptionState& preemption_state = Thread::Current::preemption_state();

   // Make sure that nothing funny is going on with our preempt disable count as
   // it stands now.
   ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

   {
     // Create a disabler inside of a scope, but do not have it immediately
     // request that preemption be disabled.  Our count should still be zero.
     AutoPreemptDisabler ap_disabler{AutoPreemptDisabler::Defer};
     ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

     // Now explicitly disable.  Our count should go to 1.
     ap_disabler.Disable();
     ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

     // Do it again, our count should remain at 1.
     ap_disabler.Disable();
     ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

     {
       // Make another inside of a new scope.  Our count should remain at 1 until
       // we explicitly use the new instance to disable preemption.
       AutoPreemptDisabler ap_disabler2{AutoPreemptDisabler::Defer};
       ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

       ap_disabler2.Disable();
       ASSERT_EQ(2u, preemption_state.PreemptDisableCount());
     }  // Let it go out of scope, we should drop down to a count of 1.

     ASSERT_EQ(1u, preemption_state.PreemptDisableCount());
   }  // Allow the original to go out of scope.  This should get us back down to a count of 0.

   ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

   // Next, do a similar test, but this time with the version which automatically
   // begins life with preemption disabled.
   {
     AutoPreemptDisabler ap_disabler;
     ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

     // Attempting to call disable should do nothing.
     ap_disabler.Disable();
     ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

     {
       // Add a second.  Watch the count go up as it comes into scope, and back
       // down again when it goes out.
       AutoPreemptDisabler ap_disabler2;
       ASSERT_EQ(2u, preemption_state.PreemptDisableCount());
     }

     ASSERT_EQ(1u, preemption_state.PreemptDisableCount());
   }  // Allow the original to go out of scope.  This should get us back down to a count of 0.

   ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

   END_TEST;
 }

 UNITTEST_START_TESTCASE(preempt_disable_tests)
 UNITTEST("test_in_timer_callback", test_in_timer_callback)
 UNITTEST("test_inc_dec_disable_counts", test_inc_dec_disable_counts)
 UNITTEST("test_decrement_clears_preempt_pending", test_decrement_clears_preempt_pending)
 UNITTEST("test_blocking_clears_preempt_pending", test_blocking_clears_preempt_pending)
 UNITTEST("test_interrupt_preserves_preempt_pending", test_interrupt_preserves_preempt_pending)
 UNITTEST("test_interrupt_clears_preempt_pending", test_interrupt_clears_preempt_pending)
 UNITTEST("test_interrupt_with_preempt_disable", test_interrupt_with_preempt_disable)
 UNITTEST("test_interrupt_with_resched_disable", test_interrupt_with_resched_disable)
 UNITTEST("test_auto_preempt_disabler", test_auto_preempt_disabler)
 UNITTEST_END_TESTCASE(preempt_disable_tests, "preempt_disable_tests", "preempt_disable_tests")
	// Copyright 2018 The Fuchsia Authors
	//
	// Use of this source code is governed by a MIT-style
	// license that can be found in the LICENSE file or at
	// https://opensource.org/licenses/MIT

	#include <lib/unittest/unittest.h>
	#include <platform.h>

	#include <kernel/auto_preempt_disabler.h>
	#include <kernel/event.h>
	#include <kernel/interrupt.h>
	#include <kernel/scheduler.h>
	#include <kernel/thread_lock.h>
	#include <kernel/timer.h>
	#include <ktl/atomic.h>

	// Test that preempt_disable is set for timer callbacks and that, in this
	// context, preempt_pending will get set by some functions.
	static void timer_callback_func(Timer* timer, zx_time_t now, void* arg) {
	Event* event = (Event*)arg;

	// Timer callbacks should be called in interrupt context with
	// preempt_disable set.
	ASSERT(arch_ints_disabled());
	ASSERT(arch_blocking_disallowed());
	PreemptionState& preemption_state = Thread::Current::preemption_state();
	ASSERT(preemption_state.PreemptDisableCount() > 0);

	// Save and restore the value of preempt_pending so that we can test
	// other functions' behavior with preempt_pending==false. It is
	// possible that preempt_pending is true now: it might have been set by
	// another timer callback.
	bool old_preempt_pending = preemption_state.preempt_pending();

	// Test that Scheduler::Reschedule() sets the preempt_pending flag when
	// preempt_disable is set.
	preemption_state.preempt_pending() = false;
	{
	Guard<SpinLock, IrqSave> guard{ThreadLock::Get()};
	Scheduler::Reschedule();
	}
	ASSERT(preemption_state.preempt_pending());

	// Test that preemption_state.PreemptSetPending() sets the preempt_pending
	// flag.
	preemption_state.preempt_pending() = false;
	preemption_state.PreemptSetPending();
	ASSERT(preemption_state.preempt_pending());

	// Restore value.
	preemption_state.preempt_pending() = old_preempt_pending;

	event->Signal();
	}

	// Schedule a timer callback and wait for it to complete. Most of the
	// testing is done in the timer callback.
	static bool test_in_timer_callback() {
	BEGIN_TEST;

	Event event;

	Timer timer;
	timer.Set(Deadline::no_slack(0), timer_callback_func, &event);

	ASSERT_EQ(event.Wait(), ZX_OK);

	END_TEST;
	}

	// Test incrementing and decrementing the preempt_disable and
	// resched_disable counts.
	static bool test_inc_dec_disable_counts() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	// Test initial conditions.
	ASSERT_EQ(preemption_state.PreemptDisableCount(), 0u);
	ASSERT_EQ(preemption_state.EagerReschedDisableCount(), 0u);
	// While preemption is allowed, a preemption should not be pending.
	ASSERT_EQ(preemption_state.preempt_pending(), false);

	// Test incrementing and decrementing of preempt_disable.
	preemption_state.PreemptDisable();
	EXPECT_EQ(preemption_state.PreemptDisableCount(), 1u);
	preemption_state.PreemptReenable();
	EXPECT_EQ(preemption_state.PreemptDisableCount(), 0u);

	// Test incrementing and decrementing of resched_disable.
	preemption_state.EagerReschedDisable();
	EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 1u);
	preemption_state.EagerReschedReenable();
	EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 0u);

	// Test nesting: multiple increments and decrements.
	preemption_state.PreemptDisable();
	preemption_state.PreemptDisable();
	EXPECT_EQ(preemption_state.PreemptDisableCount(), 2u);
	preemption_state.PreemptReenable();
	preemption_state.PreemptReenable();
	EXPECT_EQ(preemption_state.PreemptDisableCount(), 0u);

	// Test nesting: multiple increments and decrements.
	preemption_state.EagerReschedDisable();
	preemption_state.EagerReschedDisable();
	EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 2u);
	preemption_state.EagerReschedReenable();
	preemption_state.EagerReschedReenable();
	EXPECT_EQ(preemption_state.EagerReschedDisableCount(), 0u);

	END_TEST;
	}

	static bool test_decrement_clears_preempt_pending() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	// Test that preemption_state.PreemptReenable() clears preempt_pending.
	preemption_state.PreemptDisable();
	Thread::Current::Reschedule();
	// It should not be possible for an interrupt handler to block or
	// otherwise cause a reschedule before our preemption_state.PreemptReenable().
	EXPECT_EQ(preemption_state.preempt_pending(), true);
	preemption_state.PreemptReenable();
	EXPECT_EQ(preemption_state.preempt_pending(), false);

	// Test that preemption_state.ReschedReenable() clears preempt_pending.
	preemption_state.EagerReschedDisable();
	interrupt_saved_state_t int_state = arch_interrupt_save();
	Thread::Current::Reschedule();
	// Read preempt_pending with interrupts disabled because otherwise an
	// interrupt handler could set it to false.
	EXPECT_EQ(preemption_state.preempt_pending(), true);
	arch_interrupt_restore(int_state);
	preemption_state.EagerReschedReenable();
	EXPECT_EQ(preemption_state.preempt_pending(), false);

	END_TEST;
	}

	static bool test_blocking_clears_preempt_pending() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	// It is OK to block while preemption is disabled. In this case,
	// blocking should clear preempt_pending.
	preemption_state.PreemptDisable();
	Thread::Current::Reschedule();
	EXPECT_EQ(preemption_state.preempt_pending(), true);
	interrupt_saved_state_t int_state = arch_interrupt_save();
	Thread::Current::SleepRelative(ZX_MSEC(10));
	// Read preempt_pending with interrupts disabled because otherwise an
	// interrupt handler could set it to true.
	EXPECT_EQ(preemption_state.preempt_pending(), false);
	arch_interrupt_restore(int_state);
	preemption_state.PreemptReenable();

	// It is OK to block while rescheduling is disabled. In this case,
	// blocking should clear preempt_pending.
	preemption_state.EagerReschedDisable();
	Thread::Current::Reschedule();
	Thread::Current::SleepRelative(ZX_MSEC(10));
	EXPECT_EQ(preemption_state.preempt_pending(), false);
	preemption_state.EagerReschedReenable();

	END_TEST;
	}

	// Test that preempt_pending is preserved across an interrupt handler when
	// resched_disable is set and when the interrupt handler does not cause a
	// preemption. This tests the int_handler_start()/finish() routines rather
	// than the full interrupt handler.
	static bool test_interrupt_preserves_preempt_pending() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	preemption_state.EagerReschedDisable();
	// Do this with interrupts disabled so that a real interrupt does not
	// clear preempt_pending.
	interrupt_saved_state_t int_state = arch_interrupt_save();
	Thread::Current::Reschedule();

	// Simulate an interrupt handler invocation.
	int_handler_saved_state_t state;
	int_handler_start(&state);
	EXPECT_EQ(preemption_state.PreemptDisableCount(), 1u);
	bool do_preempt = int_handler_finish(&state);

	EXPECT_EQ(do_preempt, false);
	EXPECT_EQ(preemption_state.preempt_pending(), true);
	arch_interrupt_restore(int_state);
	preemption_state.EagerReschedReenable();
	EXPECT_EQ(preemption_state.preempt_pending(), false);

	END_TEST;
	}

	// Test that resched_disable does not prevent preemption by an interrupt
	// handler.
	static bool test_interrupt_clears_preempt_pending() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	preemption_state.EagerReschedDisable();
	Thread::Current::Reschedule();
	// Spin until we detect that we have been preempted. preempt_pending
	// should eventually get set to false because the scheduler should
	// preempt this thread.
	while (preemption_state.preempt_pending()) {
	}
	preemption_state.EagerReschedReenable();

	END_TEST;
	}

	// Timer callback used for testing.
	static void timer_set_preempt_pending(Timer* timer, zx_time_t now, void* arg) {
	auto* timer_ran = reinterpret_cast<ktl::atomic<bool>*>(arg);

	Thread::Current::preemption_state().PreemptSetPending();
	timer_ran->store(true);
	}

	static bool test_interrupt_with_preempt_disable() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	// Test that interrupt handlers honor preempt_disable.
	//
	// We test that by setting a timer callback that will set
	// preempt_pending from inside an interrupt handler. preempt_pending
	// should remain set after the interrupt handler returns.
	//
	// This assumes that timer_set() will run the callback on the same CPU
	// that we invoked it from. This also assumes that we don't
	// accidentally call any blocking operations that cause our thread to
	// be rescheduled to another CPU.
	preemption_state.PreemptDisable();
	ktl::atomic<bool> timer_ran(false);
	Timer timer;
	const Deadline deadline = Deadline::after(ZX_USEC(100));
	timer.Set(deadline, timer_set_preempt_pending, reinterpret_cast<void*>(&timer_ran));
	// Spin until timer_ran is set by the interrupt handler.
	while (!timer_ran.load()) {
	}
	EXPECT_EQ(preemption_state.preempt_pending(), true);
	preemption_state.PreemptReenable();

	END_TEST;
	}

	static bool test_interrupt_with_resched_disable() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	// Test that resched_disable does not defer preemptions by interrupt
	// handlers.
	//
	// This assumes that timer_set() will run the callback on the same CPU
	// that we invoked it from.
	preemption_state.EagerReschedDisable();
	ktl::atomic<bool> timer_ran(false);
	Timer timer;
	const Deadline deadline = Deadline::after(ZX_USEC(100));
	timer.Set(deadline, timer_set_preempt_pending, reinterpret_cast<void*>(&timer_ran));
	// Spin until timer_ran is set by the interrupt handler.
	while (!timer_ran.load()) {
	}
	// preempt_pending should be reset to false either on returning from
	// our timer interrupt, or by some other preemption.
	EXPECT_EQ(preemption_state.preempt_pending(), false);
	preemption_state.EagerReschedReenable();

	END_TEST;
	}

	static bool test_auto_preempt_disabler() {
	BEGIN_TEST;

	PreemptionState& preemption_state = Thread::Current::preemption_state();

	// Make sure that nothing funny is going on with our preempt disable count as
	// it stands now.
	ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

	{
	// Create a disabler inside of a scope, but do not have it immediately
	// request that preemption be disabled. Our count should still be zero.
	AutoPreemptDisabler ap_disabler{AutoPreemptDisabler::Defer};
	ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

	// Now explicitly disable. Our count should go to 1.
	ap_disabler.Disable();
	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

	// Do it again, our count should remain at 1.
	ap_disabler.Disable();
	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

	{
	// Make another inside of a new scope. Our count should remain at 1 until
	// we explicitly use the new instance to disable preemption.
	AutoPreemptDisabler ap_disabler2{AutoPreemptDisabler::Defer};
	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

	ap_disabler2.Disable();
	ASSERT_EQ(2u, preemption_state.PreemptDisableCount());
	} // Let it go out of scope, we should drop down to a count of 1.

	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());
	} // Allow the original to go out of scope. This should get us back down to a count of 0.

	ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

	// Next, do a similar test, but this time with the version which automatically
	// begins life with preemption disabled.
	{
	AutoPreemptDisabler ap_disabler;
	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

	// Attempting to call disable should do nothing.
	ap_disabler.Disable();
	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());

	{
	// Add a second. Watch the count go up as it comes into scope, and back
	// down again when it goes out.
	AutoPreemptDisabler ap_disabler2;
	ASSERT_EQ(2u, preemption_state.PreemptDisableCount());
	}

	ASSERT_EQ(1u, preemption_state.PreemptDisableCount());
	} // Allow the original to go out of scope. This should get us back down to a count of 0.

	ASSERT_EQ(0u, preemption_state.PreemptDisableCount());

	END_TEST;
	}

	UNITTEST_START_TESTCASE(preempt_disable_tests)
	UNITTEST("test_in_timer_callback", test_in_timer_callback)
	UNITTEST("test_inc_dec_disable_counts", test_inc_dec_disable_counts)
	UNITTEST("test_decrement_clears_preempt_pending", test_decrement_clears_preempt_pending)
	UNITTEST("test_blocking_clears_preempt_pending", test_blocking_clears_preempt_pending)
	UNITTEST("test_interrupt_preserves_preempt_pending", test_interrupt_preserves_preempt_pending)
	UNITTEST("test_interrupt_clears_preempt_pending", test_interrupt_clears_preempt_pending)
	UNITTEST("test_interrupt_with_preempt_disable", test_interrupt_with_preempt_disable)
	UNITTEST("test_interrupt_with_resched_disable", test_interrupt_with_resched_disable)
	UNITTEST("test_auto_preempt_disabler", test_auto_preempt_disabler)
	UNITTEST_END_TESTCASE(preempt_disable_tests, "preempt_disable_tests", "preempt_disable_tests")