adjust memory order in Semaphore::signalSlow

Summary: `signalSlow()` updates `tokens_` with a relaxed write when the wait list is empty. This won't synchronize with the acquire loads performed by calls to `wait()`. TSAN detects this: D66112566.

Reviewed By: yfeldblum

Differential Revision: D66114255

fbshipit-source-id: ce6f937b58fe0d004c1d4582db7e2cc3c1bfd6d6

folly/fibers/Semaphore.cpp

@@ -35,7 +35,7 @@ bool Semaphore::signalSlow() {
       // If the waitlist is now empty, ensure the token count increments
       // No need for CAS here as we will always be under the mutex
       CHECK(tokens_.compare_exchange_strong(
-          testVal, testVal + 1, std::memory_order_relaxed));
+          testVal, testVal + 1, std::memory_order_release));
       return true;
     }
     waiter = &waitList.front();

folly/fibers/test/BUCK

@@ -72,3 +72,15 @@ cpp_binary(
         "//folly/init:init",
     ],
 )
+
+cpp_unittest(
+    name = "semaphore_test",
+    srcs = ["SemaphoreTest.cpp"],
+    headers = [],
+    deps = [
+        "//folly/fibers:semaphore",
+        "//folly/portability:gtest",
+        "//folly/synchronization:relaxed_atomic",
+        "//folly/synchronization/detail:sleeper",
+    ],
+)

folly/fibers/test/SemaphoreTest.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/fibers/Semaphore.h>
+#include <folly/portability/GTest.h>
+#include <folly/synchronization/RelaxedAtomic.h>
+#include <folly/synchronization/detail/Sleeper.h>
+
+using namespace folly::fibers;
+
+TEST(SemaphoreTest, MessagePassing) {
+  int data = 0;
+  Semaphore sem{0};
+
+  // Provides no memory ordering: just used to reproduce conditions for a
+  // bug caught by TSAN in an earlier version of the implementation.
+  folly::relaxed_atomic<bool> signalled{false};
+
+  std::thread t{[&]() {
+    folly::detail::Sleeper sleeper;
+    while (!signalled) {
+      sleeper.wait();
+    }
+    sem.wait();
+    EXPECT_NE(0, data);
+  }};
+
+  data = 1;
+  sem.signal();
+  signalled = true;
+
+  t.join();
+}