Revert D17908896: Fix deadlock in Folly RCU

Differential Revision:
D17908896

Original commit changeset: 85847fc17f4a

fbshipit-source-id: 9b2bd8c11881cad18cffea0e14349e25d743b150

folly/synchronization/Rcu-inl.h

@@ -89,21 +89,14 @@ void rcu_domain<Tag>::retire(list_node* node) noexcept {
   if (time > syncTime + syncTimePeriod_ &&
       syncTime_.compare_exchange_strong(
           syncTime, time, std::memory_order_relaxed)) {
-    // Starting doing the sync work. However it's possible that someone else
-    // is already assigned, so add the work check here as well, and move on if
-    // the work is assigned.
-    auto target = version_.load(std::memory_order_acquire) + 1;
-    auto work = work_.load(std::memory_order_acquire);
-    if (work < target && work_.compare_exchange_strong(work, target)) {
-      list_head finished;
-      {
-        std::lock_guard<std::mutex> g(syncMutex_);
-        half_sync(false, finished);
-      }
-      // callbacks are called outside of syncMutex_
-      finished.forEach(
-          [&](list_node* item) { executor_->add(std::move(item->cb_)); });
+    list_head finished;
+    {
+      std::lock_guard<std::mutex> g(syncMutex_);
+      half_sync(false, finished);
     }
+    // callbacks are called outside of syncMutex_
+    finished.forEach(
+        [&](list_node* item) { executor_->add(std::move(item->cb_)); });
   }
 }
 

folly/synchronization/test/RcuTest.cpp

@@ -25,8 +25,6 @@
 #include <folly/Random.h>
 #include <folly/portability/GFlags.h>
 #include <folly/portability/GTest.h>
-#include <folly/synchronization/Baton.h>
-#include <folly/synchronization/test/Barrier.h>
 
 using namespace folly;
 
@@ -291,34 +289,3 @@ TEST(RcuTest, RcuObjBase) {
   synchronize_rcu();
   EXPECT_TRUE(retired);
 }
-
-namespace folly {
-
-TEST(RcuTest, DeadLock) {
-  // The test tries to reproduce the potential deadlock caused by 2 threads.
-  // T1 does rcu_read_lock(), T2 starts synchronization_rcu(), T1 tries to
-  // call_rcu(). Which made deadlock happens.
-
-  // Use fresh RcuTag and domain to consistantly reproduce the deadlock.
-  struct FreshTag;
-  rcu_domain<FreshTag> fresh_domain;
-
-  // Run the test several times to guarantee the deadlock sequence happens.
-  for (int i = 0; i < 5; i++) {
-    folly::test::Barrier sync_ready(2);
-    std::thread t1([&] {
-      folly::rcu_reader_domain guard(&fresh_domain);
-      sync_ready.wait();
-      folly::rcu_retire(new std::string(), {}, &fresh_domain);
-    });
-    std::thread t2([&] {
-      sync_ready.wait();
-      folly::synchronize_rcu(&fresh_domain);
-    });
-
-    t1.join();
-    t2.join();
-  }
-}
-
-} // namespace folly