fix deadlock in RCU retire when rcu_reader active

Summary:
Every 3.2 seconds RCU retire tries to opportunistically
drain the queue of pending work.  If this happens while
the current thread is holding an rcu_reader and another
thread is currently running synchronize(), a deadlock will
occur.  This diff adds a unit test for the behavior and
fixes the problem.

Test Plan:
* unit test that reproduces the problem
* unit test passes with the fix

CMakeLists.txt

@@ -822,6 +822,7 @@ if (BUILD_TESTS)
       TEST baton_test SOURCES BatonTest.cpp
       TEST call_once_test SOURCES CallOnceTest.cpp
       TEST lifo_sem_test SOURCES LifoSemTests.cpp
+      TEST rcu_test SOURCES RcuTest.cpp
       TEST rw_spin_lock_test SOURCES RWSpinLockTest.cpp
       TEST semaphore_test SOURCES SemaphoreTest.cpp
 

folly/synchronization/Rcu-inl.h

@@ -91,8 +91,12 @@ void rcu_domain<Tag>::retire(list_node* node) noexcept {
           syncTime, time, std::memory_order_relaxed)) {
     list_head finished;
     {
-      std::lock_guard<std::mutex> g(syncMutex_);
-      half_sync(false, finished);
+      // synchronize() blocks while holding syncMutex_,
+      // so we must not wait for syncMutex_
+      std::unique_lock<std::mutex> g(syncMutex_, std::try_to_lock);
+      if (g) {
+        half_sync(false, finished);
+      }
     }
     // callbacks are called outside of syncMutex_
     finished.forEach(

folly/synchronization/test/RcuTest.cpp

@@ -308,3 +308,26 @@ TEST(RcuTest, Tsan) {
   t2.join();
   EXPECT_EQ(data, 2);
 }
+
+TEST(RcuTest, RetireUnderLock) {
+  using namespace std::chrono;
+
+  // syncTimePeriod is 3.2s
+  auto deadline = steady_clock::now() + seconds(4);
+
+  std::thread t1([&] {
+    while (steady_clock::now() < deadline) {
+      synchronize_rcu();
+    }
+  });
+
+  std::thread t2([&] {
+    while (steady_clock::now() < deadline) {
+      rcu_reader g;
+      rcu_default_domain()->call([] {});
+    }
+  });
+
+  t1.join();
+  t2.join();
+}