tests/locks-test.cc - 3p/cheriot-rtos - Git at Google

 // Copyright Microsoft and CHERIoT Contributors.
 // SPDX-License-Identifier: MIT

 #define TEST_NAME "Locks"
 #include "tests.hh"
 #include <cheri.hh>
 #include <errno.h>
 #include <locks.hh>
 #include <thread.h>
 #include <thread_pool.h>

 using namespace CHERI;
 using namespace thread_pool;

 namespace
 {

 	FlagLock                  flagLock;
 	FlagLockPriorityInherited flagLockPriorityInherited;
 	TicketLock                ticketLock;

 	cheriot::atomic<bool> modified;
 	cheriot::atomic<int>  counter;

 	/**
 	 * Test that a lock actually provides mutual exclusion.
 	 */
 	template<typename Lock>
 	void test_lock(Lock &lock)
 	{
 		modified = false;
 		{
 			LockGuard g{lock};
 			async([&]() {
 				LockGuard g{lock};
 				modified = true;
 			});
 			sleep(2);
 			TEST(modified == false,
 			     "flag concurrently modified while lock is held!");
 		}
 		sleep(1);
 		while (!modified)
 		{
 			debug_log("Other thread not finished, yielding");
 			sleep(1);
 		}
 	}

 	/// Test that try_lock fails after a timeout
 	template<typename Lock>
 	void test_trylock(Lock &lock)
 	{
 		LockGuard g{lock};
 		debug_log("Trying to acquire already-held lock in {}",
 		          __PRETTY_FUNCTION__);
 		Timeout t{1};
 		TEST(lock.try_lock(&t) == false,
 		     "Trying to acquire lock spuriously succeeded");
 		if constexpr (!std::is_same_v<Lock, FlagLockPriorityInherited>)
 		{
 #ifndef SIMULATION
 			TEST(t.elapsed >= 1, "Sleep slept for {} ticks", t.elapsed);
 #endif
 		}
 	}

 	/**
 	 * Test that destructing a lock automatically wakes up all waiters,
 	 * failing them to acquire the lock.
 	 */
 	template<typename Lock>
 	void test_destruct_lock_wake_up(Lock &lock)
 	{
 		modified = false;

 		Timeout t{1};
 		TEST(lock.try_lock(&t), "Failed to acquire uncontended lock");

 		// Try to acquire the lock in a background thread
 		async([&]() {
 			// Make sure that we don't prevent the thread pool
 			// making progress if this test fails.
 			// The generous timer makes sure that we reach the
 			// modified == true assert before the timeout.
 			Timeout t2{25};
 			TEST(lock.try_lock(&t2) == false,
 			     "Lock acquisition should not succeed!");

 			// When the lock is upgraded in destruction mode,
 			// `lock` will return failure.
 			modified = true;
 		});

 		// Give the thread a chance to run
 		sleep(1);

 		// Upgrade the lock to destruction mode
 		lock.upgrade_for_destruction();

 		// Give the waiter a chance to wake up
 		sleep(5);

 		// Check that the destruction mode woke up the waiters
 		TEST(modified == true, "Destruction mode did not wake up waiters!");

 		// Reset the lock in case other tests use it.
 		// Note: in practice, lock.upgrade_for_destruction() would be
 		// followed by a free() operation, not by unlock(). However
 		// unlock() comes handy here to remove the destruction flag to
 		// let other tests run properly with the same lock object
 		lock.unlock();
 	}

 	/**
 	 * Test that unlocking a flag lock works: once a lock is released, it
 	 * can be re-acquired. This will fail if the unlock function does not
 	 * properly clear bits.
 	 */
 	void test_flaglock_unlock()
 	{
 		// Check for the case where a lock was released with the
 		// waiters bit set. Do not check the simple case without
 		// waiters as this is covered by other tests (at least
 		// `test_lock`).
 		Timeout t{5};
 		TEST(flagLock.try_lock(&t), "Failed to acquire uncontended lock");
 		counter = 0;
 		async([&]() {
 			Timeout t2{1};
 			TEST(flagLock.try_lock(&t2) == false,
 			     "Lock acquisition should not succeed!");
 			counter++;
 		});
 		do
 		{
 			debug_log("Other thread not finished, yielding");
 			sleep(1);
 		} while (counter.load() == 0);
 		flagLock.unlock();

 		TEST(flagLock.try_lock(&t),
 		     "Failed to acquire uncontended lock after unlock when the waiters "
 		     "bit was previously set");
 		flagLock.unlock();
 	}

 	/**
 	 * Test that a lock with the destruction bit set cannot be acquired
 	 * anymore.
 	 *
 	 * Note: here, plug at the C API to be able to check C error codes.
 	 */
 	void test_destruct_flag_lock_acquire()
 	{
 		static FlagLockState flagLockState;
 		static FlagLockState priorityFlagLockState;

 		// Upgrade the lock to destruction mode. No need to acquire the
 		// lock for that.
 		flaglock_upgrade_for_destruction(&flagLockState);

 		// Check that we now fail to grab the lock with the right error
 		Timeout t{5};
 		TEST(flaglock_trylock(&t, &flagLockState) == -ENOENT,
 		     "Acquiring the lock did not fail with -ENOENT although it is in "
 		     "destruction mode");

 		// Now, do the same tests with the priority inheriting flag lock
 		flaglock_upgrade_for_destruction(&priorityFlagLockState);

 		TEST(flaglock_priority_inheriting_trylock(&t, &priorityFlagLockState) ==
 		       -ENOENT,
 		     "Acquiring the lock did not fail with -ENOENT although it is in "
 		     "destruction mode");
 	}

 	/**
 	 * Test that the destruction bit is preserved when unlocking.
 	 *
 	 * Note: here, plug at the C API to be able to check C error codes.
 	 */
 	void test_destruct_flag_lock_unlock()
 	{
 		// Only test without priority inheriting for code size reasons,
 		// but both should behave identically
 		static FlagLockState flagLockState;

 		Timeout t{5};
 		int     ret = flaglock_trylock(&t, &flagLockState);
 		TEST(ret == 0, "Flag lock trylock failed with error {}", ret);

 		// Upgrade the lock to destruction mode
 		flaglock_upgrade_for_destruction(&flagLockState);

 		// Now unlock the lock
 		flaglock_unlock(&flagLockState);

 		// Check that the destruction bit is still set (by trying to
 		// grab the lock again - it should fail with -ENOENT)
 		TEST(flaglock_trylock(&t, &flagLockState) == -ENOENT,
 		     "Unlocking unsets the destruction bit of flag lock");
 	}

 	/**
 	 * Test that `get_owner_thread_id` returns the thread ID of the owner
 	 * of the lock.
 	 */
 	void test_get_owner_thread_id(FlagLockPriorityInherited &lock)
 	{
 		debug_log("Testing that `get_owner_thread_id` works.");

 		TEST(lock.get_owner_thread_id() == 0,
 		     "`get_owner_thread_id` does not return 0 when called on an unheld "
 		     "lock");

 		modified = false;
 		LockGuard g{lock};
 		uint16_t  ownerThreadId = thread_id_get();
 		TEST(lock.get_owner_thread_id() == ownerThreadId,
 		     "`get_owner_thread_id` does not return the thread ID of the lock "
 		     "owner");

 		async([ownerThreadId, &lock]() {
 			TEST(thread_id_get() != ownerThreadId,
 			     "Async has the same thread ID as the main thread");
 			TEST(lock.get_owner_thread_id() == ownerThreadId,
 			     "`get_owner_thread_id` does not return the thread ID of the "
 			     "lock owner when called from a non-owning thread");
 			modified = true;
 		});

 		sleep(1);
 		while (!modified)
 		{
 			debug_log("Other thread not finished, yielding");
 			sleep(1);
 		}
 	}

 	void test_recursive_mutex()
 	{
 		static RecursiveMutexState recursiveMutex;
 		static std::atomic<bool>   done{false};
 		debug_log("Testing recursive mutex");
 		Timeout t{5};
 		int     ret = recursivemutex_trylock(&t, &recursiveMutex);
 		TEST(ret == 0, "Recursive mutex trylock failed with error {}", ret);
 		debug_log("Aquiring recursive mutex again");
 		ret = recursivemutex_trylock(&t, &recursiveMutex);
 		TEST(ret == 0,
 		     "Recursive mutex trylock failed on mutex owned by this thread "
 		     "with error {}",
 		     ret);
 		// we don't expect above to block as there is no contention.
 		TEST(t.elapsed == 0,
 		     "Recursive mutex trylock slept for {} ticks",
 		     t.elapsed);
 		async([]() {
 			Timeout t{0};
 			debug_log(
 			  "Trying to acquire recursive mutex in other thread (timeout 0)");
 			int ret = recursivemutex_trylock(&t, &recursiveMutex);
 			TEST(ret != 0,
 			     "Recursive mutex trylock succeeded on mutex owned by another "
 			     "thread");
 			debug_log("Trying to acquire recursive mutex in other thread "
 			          "(unlimited timeout)");
 			t   = UnlimitedTimeout;
 			ret = recursivemutex_trylock(&t, &recursiveMutex);
 			TEST(ret == 0,
 			     "Recursive mutex failed after mutex was unlocked by "
 			     "another thread");
 			debug_log("Other thread acquired recursive mutex");
 			done = true;
 		});
 		// Give other thread a chance to run
 		sleep(2);
 		// Check that it hasn't acquired the lock yet as we still have it
 		TEST(
 		  done == false,
 		  "Recursive mutex trylock succeeded on mutex owned by another thread");
 		// Unlock once, should still not release the lock
 		debug_log("Releasing recurisve mutex once");
 		recursivemutex_unlock(&recursiveMutex);
 		sleep(2);
 		TEST(
 		  done == false,
 		  "Recursive mutex trylock succeeded on mutex owned by another thread");
 		debug_log("Releasing recurisve mutex again");
 		recursivemutex_unlock(&recursiveMutex);
 		sleep(2);
 		TEST(done == true,
 		     "Recursive mutex acquire failed from other thread after mutex was "
 		     "unlocked");
 	}

 	/**
 	 * Test that the ticket lock gives the ordering guarantees that it should.
 	 */
 	void test_ticket_lock_ordering()
 	{
 		counter = 0;
 		debug_log("Starting ticket-lock ordering tests");
 		{
 			LockGuard g{ticketLock};
 			async([&]() {
 				LockGuard g{ticketLock};
 				TEST(counter == 0,
 				     "Ticket lock acquired out of order, counter is {}, "
 				     "expected 0",
 				     counter.load());
 				counter = 1;
 			});
 			async([&]() {
 				sleep(5);
 				LockGuard g{ticketLock};
 				TEST(counter == 1,
 				     "Ticket lock acquired out of order, counter is {}, "
 				     "expected 1",
 				     counter.load());
 				counter = 2;
 			});
 			// Make sure both other threads are blocked on the ticket lock.
 			sleep(10);
 		}
 		// We should not be allowed to run until both of the other threads have
 		// run.
 		LockGuard g{ticketLock};
 		TEST(counter == 2,
 		     "Ticket lock acquired out of order, counter is {}, expected 2",
 		     counter.load());
 	}

 	/**
 	 * Test ticket lock behavior on overflow.
 	 *
 	 * Note: here, plug at the C API to be able to set the lock's starting
 	 * state.  Ideally we would be able to do that externally from the C++
 	 * API without setting a specific internal state, but this requires us
 	 * to loop `lock` and `unlock` for about 2^32 iterations, which takes
 	 * too long on both the simulator and the FPGA.
 	 */
 	void test_ticket_lock_overflow()
 	{
 		static TicketLockState ticketLockState;
 		counter = 0;

 		// Put the ticket lock in an unlocked state where it is about
 		// to overflow.
 		ticketLockState.current = std::numeric_limits<uint32_t>::max() - 2;
 		ticketLockState.next    = std::numeric_limits<uint32_t>::max() - 2;

 		// Take the lock on the main thread.
 		ticketlock_lock(&ticketLockState);
 		// Now we should have (current: max-2, next: max-1).

 		// Now create two more threads which will try to get hold of
 		// the lock, thereby overflowing the `next` counter.
 		async([&]() {
 			ticketlock_lock(&ticketLockState);
 			// Now we should have (current: max-2, next: max).
 			counter++;
 			ticketlock_unlock(&ticketLockState);
 			// Now we should have (current: max, next: 0).
 		});
 		async([&]() {
 			ticketlock_lock(&ticketLockState);
 			// We just overflowed.
 			// Now we should have (current: max-2, next: 0).
 			counter++;
 			ticketlock_unlock(&ticketLockState);
 			// Now we should have (current: 0, next: 0).
 		});

 		// Give the threads a chance to run and increment `next`.
 		sleep(20);

 		// Release the lock on the main thread.
 		ticketlock_unlock(&ticketLockState);
 		// Now we should have (current: max-1, next: 0).

 		// Give the threads a chance to wake up.
 		sleep(20);

 		// The final state should be (current: 0, next: 0).

 		// Check that unlocking woke up the waiters.  Subsequent tests
 		// will generally fail if this fails, because one or all of the
 		// two threads may still be live and deadlocked.
 		TEST(counter.load() == 2,
 		     "Ticket lock deadlocked because of overflow, expected 2 threads "
 		     "to wake up, got {}",
 		     counter.load());
 	}

 } // namespace

 int test_locks()
 {
 	test_lock(flagLock);
 	test_lock(flagLockPriorityInherited);
 	test_lock(ticketLock);
 	test_get_owner_thread_id(flagLockPriorityInherited);
 	test_flaglock_unlock();
 	test_trylock(flagLock);
 	test_trylock(flagLockPriorityInherited);
 	test_destruct_lock_wake_up(flagLock);
 	test_destruct_lock_wake_up(flagLockPriorityInherited);
 	test_destruct_flag_lock_acquire();
 	test_destruct_flag_lock_unlock();
 	test_ticket_lock_ordering();
 	test_ticket_lock_overflow();
 	test_recursive_mutex();
 	return 0;
 }
	// Copyright Microsoft and CHERIoT Contributors.
	// SPDX-License-Identifier: MIT

	#define TEST_NAME "Locks"
	#include "tests.hh"
	#include <cheri.hh>
	#include <errno.h>
	#include <locks.hh>
	#include <thread.h>
	#include <thread_pool.h>

	using namespace CHERI;
	using namespace thread_pool;

	namespace
	{

	FlagLock flagLock;
	FlagLockPriorityInherited flagLockPriorityInherited;
	TicketLock ticketLock;

	cheriot::atomic<bool> modified;
	cheriot::atomic<int> counter;

	/**
	* Test that a lock actually provides mutual exclusion.
	*/
	template<typename Lock>
	void test_lock(Lock &lock)
	{
	modified = false;
	{
	LockGuard g{lock};
	async([&]() {
	LockGuard g{lock};
	modified = true;
	});
	sleep(2);
	TEST(modified == false,
	"flag concurrently modified while lock is held!");
	}
	sleep(1);
	while (!modified)
	{
	debug_log("Other thread not finished, yielding");
	sleep(1);
	}
	}

	/// Test that try_lock fails after a timeout
	template<typename Lock>
	void test_trylock(Lock &lock)
	{
	LockGuard g{lock};
	debug_log("Trying to acquire already-held lock in {}",
	__PRETTY_FUNCTION__);
	Timeout t{1};
	TEST(lock.try_lock(&t) == false,
	"Trying to acquire lock spuriously succeeded");
	if constexpr (!std::is_same_v<Lock, FlagLockPriorityInherited>)
	{
	#ifndef SIMULATION
	TEST(t.elapsed >= 1, "Sleep slept for {} ticks", t.elapsed);
	#endif
	}
	}

	/**
	* Test that destructing a lock automatically wakes up all waiters,
	* failing them to acquire the lock.
	*/
	template<typename Lock>
	void test_destruct_lock_wake_up(Lock &lock)
	{
	modified = false;

	Timeout t{1};
	TEST(lock.try_lock(&t), "Failed to acquire uncontended lock");

	// Try to acquire the lock in a background thread
	async([&]() {
	// Make sure that we don't prevent the thread pool
	// making progress if this test fails.
	// The generous timer makes sure that we reach the
	// modified == true assert before the timeout.
	Timeout t2{25};
	TEST(lock.try_lock(&t2) == false,
	"Lock acquisition should not succeed!");

	// When the lock is upgraded in destruction mode,
	// `lock` will return failure.
	modified = true;
	});

	// Give the thread a chance to run
	sleep(1);

	// Upgrade the lock to destruction mode
	lock.upgrade_for_destruction();

	// Give the waiter a chance to wake up
	sleep(5);

	// Check that the destruction mode woke up the waiters
	TEST(modified == true, "Destruction mode did not wake up waiters!");

	// Reset the lock in case other tests use it.
	// Note: in practice, lock.upgrade_for_destruction() would be
	// followed by a free() operation, not by unlock(). However
	// unlock() comes handy here to remove the destruction flag to
	// let other tests run properly with the same lock object
	lock.unlock();
	}

	/**
	* Test that unlocking a flag lock works: once a lock is released, it
	* can be re-acquired. This will fail if the unlock function does not
	* properly clear bits.
	*/
	void test_flaglock_unlock()
	{
	// Check for the case where a lock was released with the
	// waiters bit set. Do not check the simple case without
	// waiters as this is covered by other tests (at least
	// `test_lock`).
	Timeout t{5};
	TEST(flagLock.try_lock(&t), "Failed to acquire uncontended lock");
	counter = 0;
	async([&]() {
	Timeout t2{1};
	TEST(flagLock.try_lock(&t2) == false,
	"Lock acquisition should not succeed!");
	counter++;
	});
	do
	{
	debug_log("Other thread not finished, yielding");
	sleep(1);
	} while (counter.load() == 0);
	flagLock.unlock();

	TEST(flagLock.try_lock(&t),
	"Failed to acquire uncontended lock after unlock when the waiters "
	"bit was previously set");
	flagLock.unlock();
	}

	/**
	* Test that a lock with the destruction bit set cannot be acquired
	* anymore.
	*
	* Note: here, plug at the C API to be able to check C error codes.
	*/
	void test_destruct_flag_lock_acquire()
	{
	static FlagLockState flagLockState;
	static FlagLockState priorityFlagLockState;

	// Upgrade the lock to destruction mode. No need to acquire the
	// lock for that.
	flaglock_upgrade_for_destruction(&flagLockState);

	// Check that we now fail to grab the lock with the right error
	Timeout t{5};
	TEST(flaglock_trylock(&t, &flagLockState) == -ENOENT,
	"Acquiring the lock did not fail with -ENOENT although it is in "
	"destruction mode");

	// Now, do the same tests with the priority inheriting flag lock
	flaglock_upgrade_for_destruction(&priorityFlagLockState);

	TEST(flaglock_priority_inheriting_trylock(&t, &priorityFlagLockState) ==
	-ENOENT,
	"Acquiring the lock did not fail with -ENOENT although it is in "
	"destruction mode");
	}

	/**
	* Test that the destruction bit is preserved when unlocking.
	*
	* Note: here, plug at the C API to be able to check C error codes.
	*/
	void test_destruct_flag_lock_unlock()
	{
	// Only test without priority inheriting for code size reasons,
	// but both should behave identically
	static FlagLockState flagLockState;

	Timeout t{5};
	int ret = flaglock_trylock(&t, &flagLockState);
	TEST(ret == 0, "Flag lock trylock failed with error {}", ret);

	// Upgrade the lock to destruction mode
	flaglock_upgrade_for_destruction(&flagLockState);

	// Now unlock the lock
	flaglock_unlock(&flagLockState);

	// Check that the destruction bit is still set (by trying to
	// grab the lock again - it should fail with -ENOENT)
	TEST(flaglock_trylock(&t, &flagLockState) == -ENOENT,
	"Unlocking unsets the destruction bit of flag lock");
	}

	/**
	* Test that `get_owner_thread_id` returns the thread ID of the owner
	* of the lock.
	*/
	void test_get_owner_thread_id(FlagLockPriorityInherited &lock)
	{
	debug_log("Testing that `get_owner_thread_id` works.");

	TEST(lock.get_owner_thread_id() == 0,
	"`get_owner_thread_id` does not return 0 when called on an unheld "
	"lock");

	modified = false;
	LockGuard g{lock};
	uint16_t ownerThreadId = thread_id_get();
	TEST(lock.get_owner_thread_id() == ownerThreadId,
	"`get_owner_thread_id` does not return the thread ID of the lock "
	"owner");

	async([ownerThreadId, &lock]() {
	TEST(thread_id_get() != ownerThreadId,
	"Async has the same thread ID as the main thread");
	TEST(lock.get_owner_thread_id() == ownerThreadId,
	"`get_owner_thread_id` does not return the thread ID of the "
	"lock owner when called from a non-owning thread");
	modified = true;
	});

	sleep(1);
	while (!modified)
	{
	debug_log("Other thread not finished, yielding");
	sleep(1);
	}
	}

	void test_recursive_mutex()
	{
	static RecursiveMutexState recursiveMutex;
	static std::atomic<bool> done{false};
	debug_log("Testing recursive mutex");
	Timeout t{5};
	int ret = recursivemutex_trylock(&t, &recursiveMutex);
	TEST(ret == 0, "Recursive mutex trylock failed with error {}", ret);
	debug_log("Aquiring recursive mutex again");
	ret = recursivemutex_trylock(&t, &recursiveMutex);
	TEST(ret == 0,
	"Recursive mutex trylock failed on mutex owned by this thread "
	"with error {}",
	ret);
	// we don't expect above to block as there is no contention.
	TEST(t.elapsed == 0,
	"Recursive mutex trylock slept for {} ticks",
	t.elapsed);
	async([]() {
	Timeout t{0};
	debug_log(
	"Trying to acquire recursive mutex in other thread (timeout 0)");
	int ret = recursivemutex_trylock(&t, &recursiveMutex);
	TEST(ret != 0,
	"Recursive mutex trylock succeeded on mutex owned by another "
	"thread");
	debug_log("Trying to acquire recursive mutex in other thread "
	"(unlimited timeout)");
	t = UnlimitedTimeout;
	ret = recursivemutex_trylock(&t, &recursiveMutex);
	TEST(ret == 0,
	"Recursive mutex failed after mutex was unlocked by "
	"another thread");
	debug_log("Other thread acquired recursive mutex");
	done = true;
	});
	// Give other thread a chance to run
	sleep(2);
	// Check that it hasn't acquired the lock yet as we still have it
	TEST(
	done == false,
	"Recursive mutex trylock succeeded on mutex owned by another thread");
	// Unlock once, should still not release the lock
	debug_log("Releasing recurisve mutex once");
	recursivemutex_unlock(&recursiveMutex);
	sleep(2);
	TEST(
	done == false,
	"Recursive mutex trylock succeeded on mutex owned by another thread");
	debug_log("Releasing recurisve mutex again");
	recursivemutex_unlock(&recursiveMutex);
	sleep(2);
	TEST(done == true,
	"Recursive mutex acquire failed from other thread after mutex was "
	"unlocked");
	}

	/**
	* Test that the ticket lock gives the ordering guarantees that it should.
	*/
	void test_ticket_lock_ordering()
	{
	counter = 0;
	debug_log("Starting ticket-lock ordering tests");
	{
	LockGuard g{ticketLock};
	async([&]() {
	LockGuard g{ticketLock};
	TEST(counter == 0,
	"Ticket lock acquired out of order, counter is {}, "
	"expected 0",
	counter.load());
	counter = 1;
	});
	async([&]() {
	sleep(5);
	LockGuard g{ticketLock};
	TEST(counter == 1,
	"Ticket lock acquired out of order, counter is {}, "
	"expected 1",
	counter.load());
	counter = 2;
	});
	// Make sure both other threads are blocked on the ticket lock.
	sleep(10);
	}
	// We should not be allowed to run until both of the other threads have
	// run.
	LockGuard g{ticketLock};
	TEST(counter == 2,
	"Ticket lock acquired out of order, counter is {}, expected 2",
	counter.load());
	}

	/**
	* Test ticket lock behavior on overflow.
	*
	* Note: here, plug at the C API to be able to set the lock's starting
	* state. Ideally we would be able to do that externally from the C++
	* API without setting a specific internal state, but this requires us
	* to loop `lock` and `unlock` for about 2^32 iterations, which takes
	* too long on both the simulator and the FPGA.
	*/
	void test_ticket_lock_overflow()
	{
	static TicketLockState ticketLockState;
	counter = 0;

	// Put the ticket lock in an unlocked state where it is about
	// to overflow.
	ticketLockState.current = std::numeric_limits<uint32_t>::max() - 2;
	ticketLockState.next = std::numeric_limits<uint32_t>::max() - 2;

	// Take the lock on the main thread.
	ticketlock_lock(&ticketLockState);
	// Now we should have (current: max-2, next: max-1).

	// Now create two more threads which will try to get hold of
	// the lock, thereby overflowing the `next` counter.
	async([&]() {
	ticketlock_lock(&ticketLockState);
	// Now we should have (current: max-2, next: max).
	counter++;
	ticketlock_unlock(&ticketLockState);
	// Now we should have (current: max, next: 0).
	});
	async([&]() {
	ticketlock_lock(&ticketLockState);
	// We just overflowed.
	// Now we should have (current: max-2, next: 0).
	counter++;
	ticketlock_unlock(&ticketLockState);
	// Now we should have (current: 0, next: 0).
	});

	// Give the threads a chance to run and increment `next`.
	sleep(20);

	// Release the lock on the main thread.
	ticketlock_unlock(&ticketLockState);
	// Now we should have (current: max-1, next: 0).

	// Give the threads a chance to wake up.
	sleep(20);

	// The final state should be (current: 0, next: 0).

	// Check that unlocking woke up the waiters. Subsequent tests
	// will generally fail if this fails, because one or all of the
	// two threads may still be live and deadlocked.
	TEST(counter.load() == 2,
	"Ticket lock deadlocked because of overflow, expected 2 threads "
	"to wake up, got {}",
	counter.load());
	}

	} // namespace

	int test_locks()
	{
	test_lock(flagLock);
	test_lock(flagLockPriorityInherited);
	test_lock(ticketLock);
	test_get_owner_thread_id(flagLockPriorityInherited);
	test_flaglock_unlock();
	test_trylock(flagLock);
	test_trylock(flagLockPriorityInherited);
	test_destruct_lock_wake_up(flagLock);
	test_destruct_lock_wake_up(flagLockPriorityInherited);
	test_destruct_flag_lock_acquire();
	test_destruct_flag_lock_unlock();
	test_ticket_lock_ordering();
	test_ticket_lock_overflow();
	test_recursive_mutex();
	return 0;
	}