[libcxx testing] Make three locking tests more reliable

The challenge with measuring time in tests is that slow and/or busy
machines can cause tests to fail in unexpected ways. After this change,
three tests should be much more robust. The only remaining and tiny race
that I can think of is preemption after `--countDown`. That being said,
the race isn't fixable because the standard library doesn't provide a
way to count threads that are waiting to acquire a lock.

Reviewers: ldionne, EricWF, howard.hinnant, mclow.lists, #libc

Reviewed By: ldionne, #libc

Subscribers: dexonsmith, jfb, broadwaylamb, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D79406

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/lock.pass.cpp

@@ -9,8 +9,6 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
-// ALLOW_RETRIES: 2
-
 // shared_timed_mutex was introduced in macosx10.12
 // UNSUPPORTED: with_system_cxx_lib=macosx10.11
 // UNSUPPORTED: with_system_cxx_lib=macosx10.10
@@ -37,36 +35,32 @@ typedef Clock::duration duration;
 typedef std::chrono::milliseconds ms;
 typedef std::chrono::nanoseconds ns;
 
+std::atomic<bool> ready(false);
+time_point start;
 
 ms WaitTime = ms(250);
 
-// Thread sanitizer causes more overhead and will sometimes cause this test
-// to fail. To prevent this we give Thread sanitizer more time to complete the
-// test.
-#if !TEST_HAS_FEATURE(thread_sanitizer)
-ms Tolerance = ms(50);
-#else
-ms Tolerance = ms(100);
-#endif
-
-
 void f()
 {
-    time_point t0 = Clock::now();
-    m.lock();
-    time_point t1 = Clock::now();
-    m.unlock();
-    ns d = t1 - t0 - ms(250);
-    assert(d < ms(50));  // within 50ms
+  ready.store(true);
+  m.lock();
+  time_point t0 = start;
+  time_point t1 = Clock::now();
+  m.unlock();
+  assert(t0.time_since_epoch() > ms(0));
+  assert(t1 - t0 >= WaitTime);
 }
 
 int main(int, char**)
 {
-    m.lock();
-    std::thread t(f);
-    std::this_thread::sleep_for(ms(250));
-    m.unlock();
-    t.join();
+  m.lock();
+  std::thread t(f);
+  while (!ready)
+    std::this_thread::yield();
+  start = Clock::now();
+  std::this_thread::sleep_for(WaitTime);
+  m.unlock();
+  t.join();
 
   return 0;
 }

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/lock_shared.pass.cpp

@@ -9,8 +9,6 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
-// ALLOW_RETRIES: 2
-
 // shared_timed_mutex was introduced in macosx10.12
 // UNSUPPORTED: with_system_cxx_lib=macosx10.11
 // UNSUPPORTED: with_system_cxx_lib=macosx10.10
@@ -38,59 +36,68 @@ typedef Clock::duration duration;
 typedef std::chrono::milliseconds ms;
 typedef std::chrono::nanoseconds ns;
 
+std::atomic<unsigned> countDown;
+time_point readerStart; // Protected by the above mutex 'm'
+time_point writerStart; // Protected by the above mutex 'm'
 
 ms WaitTime = ms(250);
 
-// Thread sanitizer causes more overhead and will sometimes cause this test
-// to fail. To prevent this we give Thread sanitizer more time to complete the
-// test.
-#if !defined(TEST_HAS_SANITIZERS)
-ms Tolerance = ms(50);
-#else
-ms Tolerance = ms(50 * 5);
-#endif
-
-
-void f()
-{
-    time_point t0 = Clock::now();
-    m.lock_shared();
-    time_point t1 = Clock::now();
-    m.unlock_shared();
-    ns d = t1 - t0 - WaitTime;
-    assert(d < Tolerance);  // within tolerance
+void readerMustWait() {
+  --countDown;
+  m.lock_shared();
+  time_point t1 = Clock::now();
+  time_point t0 = readerStart;
+  m.unlock_shared();
+  assert(t0.time_since_epoch() > ms(0));
+  assert(t1 - t0 >= WaitTime);
 }
 
-void g()
-{
-    time_point t0 = Clock::now();
-    m.lock_shared();
-    time_point t1 = Clock::now();
-    m.unlock_shared();
-    ns d = t1 - t0;
-    assert(d < Tolerance);  // within tolerance
+void reader() {
+  --countDown;
+  m.lock_shared();
+  m.unlock_shared();
 }
 
+void writerMustWait() {
+  --countDown;
+  m.lock();
+  time_point t1 = Clock::now();
+  time_point t0 = writerStart;
+  m.unlock();
+  assert(t0.time_since_epoch() > ms(0));
+  assert(t1 - t0 >= WaitTime);
+}
 
 int main(int, char**)
 {
-    m.lock();
-    std::vector<std::thread> v;
-    for (int i = 0; i < 5; ++i)
-        v.push_back(std::thread(f));
-    std::this_thread::sleep_for(WaitTime);
-    m.unlock();
-    for (auto& t : v)
-        t.join();
-    m.lock_shared();
-    for (auto& t : v)
-        t = std::thread(g);
-    std::thread q(f);
-    std::this_thread::sleep_for(WaitTime);
-    m.unlock_shared();
-    for (auto& t : v)
-        t.join();
-    q.join();
+  int threads = 5;
+
+  countDown.store(threads);
+  m.lock();
+  std::vector<std::thread> v;
+  for (int i = 0; i < threads; ++i)
+    v.push_back(std::thread(readerMustWait));
+  while (countDown > 0)
+    std::this_thread::yield();
+  readerStart = Clock::now();
+  std::this_thread::sleep_for(WaitTime);
+  m.unlock();
+  for (auto& t : v)
+    t.join();
+
+  countDown.store(threads + 1);
+  m.lock_shared();
+  for (auto& t : v)
+    t = std::thread(reader);
+  std::thread q(writerMustWait);
+  while (countDown > 0)
+    std::this_thread::yield();
+  writerStart = Clock::now();
+  std::this_thread::sleep_for(WaitTime);
+  m.unlock_shared();
+  for (auto& t : v)
+    t.join();
+  q.join();
 
   return 0;
 }

libcxx/test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/try_lock_shared_until.pass.cpp

@@ -9,8 +9,6 @@
 // UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++98, c++03, c++11
 
-// ALLOW_RETRIES: 2
-
 // shared_timed_mutex was introduced in macosx10.12
 // UNSUPPORTED: with_system_cxx_lib=macosx10.11
 // UNSUPPORTED: with_system_cxx_lib=macosx10.10
@@ -39,58 +37,55 @@ typedef Clock::duration duration;
 typedef std::chrono::milliseconds ms;
 typedef std::chrono::nanoseconds ns;
 
-ms WaitTime = ms(250);
+ms SuccessWaitTime = ms(5000); // Some machines are busy or slow or both
+ms FailureWaitTime = ms(50);
 
-// Thread sanitizer causes more overhead and will sometimes cause this test
-// to fail. To prevent this we give Thread sanitizer more time to complete the
-// test.
-#if !defined(TEST_HAS_SANITIZERS)
-ms Tolerance = ms(50);
-#else
-ms Tolerance = ms(50 * 5);
-#endif
+// On busy or slow machines, there can be a significant delay between thread
+// creation and thread start, so we use an atomic variable to signal that the
+// thread is actually executing.
+static std::atomic<unsigned> countDown;
 
 void f1()
 {
-    time_point t0 = Clock::now();
-    assert(m.try_lock_shared_until(Clock::now() + WaitTime + Tolerance) == true);
-    time_point t1 = Clock::now();
-    m.unlock_shared();
-    ns d = t1 - t0 - WaitTime;
-    assert(d < Tolerance);  // within 50ms
+  --countDown;
+  time_point t0 = Clock::now();
+  assert(m.try_lock_shared_until(Clock::now() + SuccessWaitTime) == true);
+  time_point t1 = Clock::now();
+  m.unlock_shared();
+  assert(t1 - t0 <= SuccessWaitTime);
 }
 
 void f2()
 {
-    time_point t0 = Clock::now();
-    assert(m.try_lock_shared_until(Clock::now() + WaitTime) == false);
-    time_point t1 = Clock::now();
-    ns d = t1 - t0 - WaitTime;
-    assert(d < Tolerance);  // within tolerance
+  time_point t0 = Clock::now();
+  assert(m.try_lock_shared_until(Clock::now() + FailureWaitTime) == false);
+  assert(Clock::now() - t0 >= FailureWaitTime);
 }
 
 int main(int, char**)
 {
-    {
-        m.lock();
-        std::vector<std::thread> v;
-        for (int i = 0; i < 5; ++i)
-            v.push_back(std::thread(f1));
-        std::this_thread::sleep_for(WaitTime);
-        m.unlock();
-        for (auto& t : v)
-            t.join();
-    }
-    {
-        m.lock();
-        std::vector<std::thread> v;
-        for (int i = 0; i < 5; ++i)
-            v.push_back(std::thread(f2));
-        std::this_thread::sleep_for(WaitTime + Tolerance);
-        m.unlock();
-        for (auto& t : v)
-            t.join();
-    }
+  int threads = 5;
+  {
+    countDown.store(threads);
+    m.lock();
+    std::vector<std::thread> v;
+    for (int i = 0; i < threads; ++i)
+      v.push_back(std::thread(f1));
+    while (countDown > 0)
+      std::this_thread::yield();
+    m.unlock();
+    for (auto& t : v)
+      t.join();
+  }
+  {
+    m.lock();
+    std::vector<std::thread> v;
+    for (int i = 0; i < threads; ++i)
+      v.push_back(std::thread(f2));
+    for (auto& t : v)
+      t.join();
+    m.unlock();
+  }
 
   return 0;
 }