From 5009049e96ecc8c6f5e8d147f6c93a3d645f195b Mon Sep 17 00:00:00 2001
From: Faruk Barotov <fbarotov@meta.com>
Date: Fri, 24 Jan 2025 23:11:36 -0800
Subject: [PATCH] Fix a race in fibers::BatchSemaphore

Summary: D66114255 fixed a data race in `fibers::Semaphore` occuring due to lack of 'happens-before' signal-wait synchronization. The very same issue exists in `fibers::BatchedSemaphore` which I came across today in a flaky unit test. Hence porting the fix.

Reviewed By: Gownta

Differential Revision: D68594868

fbshipit-source-id: 752a7b15198015928a7fe040fecc8b3c9e56a889
---
 folly/fibers/SemaphoreBase.cpp           |  2 +-
 folly/fibers/test/BUCK                   |  9 ++++++
 folly/fibers/test/BatchSemaphoreTest.cpp | 38 ++++++++++++++++++++++++
 3 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 folly/fibers/test/BatchSemaphoreTest.cpp

diff --git a/folly/fibers/SemaphoreBase.cpp b/folly/fibers/SemaphoreBase.cpp
index a6ad891a7d9..5832f5e9bd8 100644
--- a/folly/fibers/SemaphoreBase.cpp
+++ b/folly/fibers/SemaphoreBase.cpp
@@ -34,7 +34,7 @@ bool SemaphoreBase::signalSlow(int64_t tokens) {
         // waitlist, ensure the token count increments. No need for CAS here as
         // we will always be under the mutex
         if (tokens_.compare_exchange_strong(
-                testVal, testVal + tokens, std::memory_order_relaxed)) {
+                testVal, testVal + tokens, std::memory_order_release)) {
           return true;
         }
         continue;
diff --git a/folly/fibers/test/BUCK b/folly/fibers/test/BUCK
index 396c9e2fd78..db1d9d1ea01 100644
--- a/folly/fibers/test/BUCK
+++ b/folly/fibers/test/BUCK
@@ -85,3 +85,12 @@ cpp_unittest(
         "//folly/synchronization/detail:sleeper",
     ],
 )
+
+cpp_unittest(
+    name = "batch_semaphore_test",
+    srcs = ["BatchSemaphoreTest.cpp"],
+    deps = [
+        "//folly/fibers:batch_semaphore",
+        "//folly/portability:gtest",
+    ],
+)
diff --git a/folly/fibers/test/BatchSemaphoreTest.cpp b/folly/fibers/test/BatchSemaphoreTest.cpp
new file mode 100644
index 00000000000..25f0d5a2f52
--- /dev/null
+++ b/folly/fibers/test/BatchSemaphoreTest.cpp
@@ -0,0 +1,38 @@
+/*
+ * 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/BatchSemaphore.h>
+#include <folly/portability/GTest.h>
+
+TEST(BatchSemaphoreTest, WaitSignalSynchronization) {
+  folly::fibers::BatchSemaphore sem{0};
+
+  int64_t data = 0;
+  folly::relaxed_atomic_bool signalled = false;
+
+  std::jthread t{[&]() {
+    while (!signalled) {
+      std::this_thread::yield();
+    }
+
+    sem.wait(1);
+    EXPECT_NE(data, 0);
+  }};
+
+  data = 1;
+  sem.signal(1);
+  signalled = true;
+}