[SDK] Fix forceflush may wait for ever (#2584)

Co-authored-by: Marc Alff <marc.alff@oracle.com>
Co-authored-by: Tom Tan <Tom.Tan@microsoft.com>

sdk/include/opentelemetry/sdk/logs/batch_log_record_processor.h

@@ -115,20 +115,25 @@ class BatchLogRecordProcessor : public LogRecordProcessor
 
     /* Important boolean flags to handle the workflow of the processor */
     std::atomic<bool> is_force_wakeup_background_worker{false};
-    std::atomic<bool> is_force_flush_pending{false};
-    std::atomic<bool> is_force_flush_notified{false};
-    std::atomic<std::chrono::microseconds::rep> force_flush_timeout_us{0};
     std::atomic<bool> is_shutdown{false};
+    std::atomic<uint64_t> force_flush_pending_sequence{0};
+    std::atomic<uint64_t> force_flush_notified_sequence{0};
+    std::atomic<std::chrono::microseconds::rep> force_flush_timeout_us{0};
+
+    // Do not use SynchronizationData() = default; here, some versions of GCC&Clang have BUGs
+    // and may not initialize the member correctly. See also
+    // https://stackoverflow.com/questions/53408962/try-to-understand-compiler-error-message-default-member-initializer-required-be
+    inline SynchronizationData() {}
   };
 
   /**
    * @brief Notify completion of shutdown and force flush. This may be called from the any thread at
    * any time
    *
-   * @param notify_force_flush Flag to indicate whether to notify force flush completion.
+   * @param notify_force_flush Sequence to indicate whether to notify force flush completion.
    * @param synchronization_data Synchronization data to be notified.
    */
-  static void NotifyCompletion(bool notify_force_flush,
+  static void NotifyCompletion(uint64_t notify_force_flush,
                                const std::unique_ptr<LogRecordExporter> &exporter,
                                const std::shared_ptr<SynchronizationData> &synchronization_data);
 

sdk/include/opentelemetry/sdk/metrics/export/periodic_exporting_metric_reader.h

@@ -6,6 +6,7 @@
 #include <atomic>
 #include <chrono>
 #include <condition_variable>
+#include <cstdint>
 #include <memory>
 #include <mutex>
 #include <thread>
@@ -50,9 +51,9 @@ class PeriodicExportingMetricReader : public MetricReader
   std::thread worker_thread_;
 
   /* Synchronization primitives */
-  std::atomic<bool> is_force_flush_pending_{false};
   std::atomic<bool> is_force_wakeup_background_worker_{false};
-  std::atomic<bool> is_force_flush_notified_{false};
+  std::atomic<uint64_t> force_flush_pending_sequence_{0};
+  std::atomic<uint64_t> force_flush_notified_sequence_{0};
   std::condition_variable cv_, force_flush_cv_;
   std::mutex cv_m_, force_flush_m_;
 };

sdk/include/opentelemetry/sdk/trace/batch_span_processor.h

@@ -5,6 +5,7 @@
 
 #include <atomic>
 #include <condition_variable>
+#include <cstdint>
 #include <memory>
 #include <mutex>
 #include <thread>
@@ -115,20 +116,25 @@ class BatchSpanProcessor : public SpanProcessor
 
     /* Important boolean flags to handle the workflow of the processor */
     std::atomic<bool> is_force_wakeup_background_worker{false};
-    std::atomic<bool> is_force_flush_pending{false};
-    std::atomic<bool> is_force_flush_notified{false};
-    std::atomic<std::chrono::microseconds::rep> force_flush_timeout_us{0};
     std::atomic<bool> is_shutdown{false};
+    std::atomic<uint64_t> force_flush_pending_sequence{0};
+    std::atomic<uint64_t> force_flush_notified_sequence{0};
+    std::atomic<std::chrono::microseconds::rep> force_flush_timeout_us{0};
+
+    // Do not use SynchronizationData() = default; here, some versions of GCC&Clang have BUGs
+    // and may not initialize the member correctly. See also
+    // https://stackoverflow.com/questions/53408962/try-to-understand-compiler-error-message-default-member-initializer-required-be
+    inline SynchronizationData() {}
   };
 
   /**
    * @brief Notify completion of shutdown and force flush. This may be called from the any thread at
    * any time
    *
-   * @param notify_force_flush Flag to indicate whether to notify force flush completion.
+   * @param notify_force_flush Sequence to indicate whether to notify force flush completion.
    * @param synchronization_data Synchronization data to be notified.
    */
-  static void NotifyCompletion(bool notify_force_flush,
+  static void NotifyCompletion(uint64_t notify_force_flush,
                                const std::unique_ptr<SpanExporter> &exporter,
                                const std::shared_ptr<SynchronizationData> &synchronization_data);
 

sdk/src/logs/batch_log_record_processor.cc

@@ -65,7 +65,7 @@ void BatchLogRecordProcessor::OnEmit(std::unique_ptr<Recordable> &&record) noexc
   {
     // signal the worker thread
     synchronization_data_->is_force_wakeup_background_worker.store(true, std::memory_order_release);
-    synchronization_data_->cv.notify_one();
+    synchronization_data_->cv.notify_all();
   }
 }
 
@@ -79,21 +79,25 @@ bool BatchLogRecordProcessor::ForceFlush(std::chrono::microseconds timeout) noex
   // Now wait for the worker thread to signal back from the Export method
   std::unique_lock<std::mutex> lk_cv(synchronization_data_->force_flush_cv_m);
 
-  synchronization_data_->is_force_flush_pending.store(true, std::memory_order_release);
+  std::uint64_t current_sequence =
+      synchronization_data_->force_flush_pending_sequence.fetch_add(1, std::memory_order_release) +
+      1;
   synchronization_data_->force_flush_timeout_us.store(timeout.count(), std::memory_order_release);
-  auto break_condition = [this]() {
+  auto break_condition = [this, current_sequence]() {
     if (synchronization_data_->is_shutdown.load() == true)
     {
       return true;
     }
 
     // Wake up the worker thread once.
-    if (synchronization_data_->is_force_flush_pending.load(std::memory_order_acquire))
+    if (synchronization_data_->force_flush_pending_sequence.load(std::memory_order_acquire) >
+        synchronization_data_->force_flush_notified_sequence.load(std::memory_order_acquire))
     {
-      synchronization_data_->cv.notify_one();
+      synchronization_data_->cv.notify_all();
     }
 
-    return synchronization_data_->is_force_flush_notified.load(std::memory_order_acquire);
+    return synchronization_data_->force_flush_notified_sequence.load(std::memory_order_acquire) >=
+           current_sequence;
   };
 
   // Fix timeout to meet requirement of wait_for
@@ -110,35 +114,22 @@ bool BatchLogRecordProcessor::ForceFlush(std::chrono::microseconds timeout) noex
   bool result = false;
   while (!result && timeout_steady > std::chrono::steady_clock::duration::zero())
   {
-    // When is_force_flush_notified.store(true) and force_flush_cv.notify_all() is called
-    // between is_force_flush_pending.load() and force_flush_cv.wait(). We must not wait
-    // for ever
+    // When force_flush_notified_sequence.compare_exchange_strong(...) and
+    // force_flush_cv.notify_all() is called between force_flush_pending_sequence.load(...) and
+    // force_flush_cv.wait(). We must not wait for ever
     std::chrono::steady_clock::time_point start_timepoint = std::chrono::steady_clock::now();
-    result = synchronization_data_->force_flush_cv.wait_for(lk_cv, scheduled_delay_millis_,
-                                                            break_condition);
-    timeout_steady -= std::chrono::steady_clock::now() - start_timepoint;
-  }
+    std::chrono::microseconds wait_timeout                = scheduled_delay_millis_;
 
-  // If it's already signaled, we must wait util notified.
-  // We use a spin lock here
-  if (false ==
-      synchronization_data_->is_force_flush_pending.exchange(false, std::memory_order_acq_rel))
-  {
-    for (int retry_waiting_times = 0;
-         false == synchronization_data_->is_force_flush_notified.load(std::memory_order_acquire);
-         ++retry_waiting_times)
+    if (wait_timeout > timeout_steady)
     {
-      opentelemetry::common::SpinLockMutex::fast_yield();
-      if ((retry_waiting_times & 127) == 127)
-      {
-        std::this_thread::yield();
-      }
+      wait_timeout = std::chrono::duration_cast<std::chrono::microseconds>(timeout_steady);
     }
+    result = synchronization_data_->force_flush_cv.wait_for(lk_cv, wait_timeout, break_condition);
+    timeout_steady -= std::chrono::steady_clock::now() - start_timepoint;
   }
 
-  synchronization_data_->is_force_flush_notified.store(false, std::memory_order_release);
-
-  return result;
+  return synchronization_data_->force_flush_notified_sequence.load(std::memory_order_acquire) >=
+         current_sequence;
 }
 
 void BatchLogRecordProcessor::DoBackgroundWork()
@@ -182,8 +173,8 @@ void BatchLogRecordProcessor::Export()
   {
     std::vector<std::unique_ptr<Recordable>> records_arr;
     size_t num_records_to_export;
-    bool notify_force_flush =
-        synchronization_data_->is_force_flush_pending.exchange(false, std::memory_order_acq_rel);
+    std::uint64_t notify_force_flush =
+        synchronization_data_->force_flush_pending_sequence.load(std::memory_order_acquire);
     if (notify_force_flush)
     {
       num_records_to_export = buffer_.size();
@@ -217,7 +208,7 @@ void BatchLogRecordProcessor::Export()
 }
 
 void BatchLogRecordProcessor::NotifyCompletion(
-    bool notify_force_flush,
+    std::uint64_t notify_force_flush,
     const std::unique_ptr<LogRecordExporter> &exporter,
     const std::shared_ptr<SynchronizationData> &synchronization_data)
 {
@@ -226,7 +217,8 @@ void BatchLogRecordProcessor::NotifyCompletion(
     return;
   }
 
-  if (notify_force_flush)
+  if (notify_force_flush >
+      synchronization_data->force_flush_notified_sequence.load(std::memory_order_acquire))
   {
     if (exporter)
     {
@@ -236,8 +228,15 @@ void BatchLogRecordProcessor::NotifyCompletion(
           std::chrono::microseconds::zero());
       exporter->ForceFlush(timeout);
     }
-    synchronization_data->is_force_flush_notified.store(true, std::memory_order_release);
-    synchronization_data->force_flush_cv.notify_one();
+
+    std::uint64_t notified_sequence =
+        synchronization_data->force_flush_notified_sequence.load(std::memory_order_acquire);
+    while (notify_force_flush > notified_sequence)
+    {
+      synchronization_data->force_flush_notified_sequence.compare_exchange_strong(
+          notified_sequence, notify_force_flush, std::memory_order_acq_rel);
+      synchronization_data->force_flush_cv.notify_all();
+    }
   }
 }
 
@@ -246,7 +245,8 @@ void BatchLogRecordProcessor::DrainQueue()
   while (true)
   {
     if (buffer_.empty() &&
-        false == synchronization_data_->is_force_flush_pending.load(std::memory_order_acquire))
+        synchronization_data_->force_flush_pending_sequence.load(std::memory_order_acquire) <=
+            synchronization_data_->force_flush_notified_sequence.load(std::memory_order_acquire))
     {
       break;
     }
@@ -285,7 +285,7 @@ bool BatchLogRecordProcessor::Shutdown(std::chrono::microseconds timeout) noexce
   if (worker_thread_.joinable())
   {
     synchronization_data_->is_force_wakeup_background_worker.store(true, std::memory_order_release);
-    synchronization_data_->cv.notify_one();
+    synchronization_data_->cv.notify_all();
     worker_thread_.join();
   }
 

sdk/src/metrics/export/periodic_exporting_metric_reader.cc

@@ -90,6 +90,7 @@ bool PeriodicExportingMetricReader::CollectAndExportOnce()
   });
 
   std::future_status status;
+  std::uint64_t notify_force_flush = force_flush_pending_sequence_.load(std::memory_order_acquire);
   do
   {
     status = future_receive.wait_for(std::chrono::milliseconds(export_timeout_millis_));
@@ -99,12 +100,13 @@ bool PeriodicExportingMetricReader::CollectAndExportOnce()
       break;
     }
   } while (status != std::future_status::ready);
-  bool notify_force_flush = is_force_flush_pending_.exchange(false, std::memory_order_acq_rel);
-  if (notify_force_flush)
+
+  std::uint64_t notified_sequence = force_flush_notified_sequence_.load(std::memory_order_acquire);
+  while (notify_force_flush > notified_sequence)
   {
-    std::unique_lock<std::mutex> lk(force_flush_m_);
-    is_force_flush_notified_.store(true, std::memory_order_release);
-    force_flush_cv_.notify_one();
+    force_flush_notified_sequence_.compare_exchange_strong(notified_sequence, notify_force_flush,
+                                                           std::memory_order_acq_rel);
+    force_flush_cv_.notify_all();
   }
 
   return true;
@@ -113,24 +115,27 @@ bool PeriodicExportingMetricReader::CollectAndExportOnce()
 bool PeriodicExportingMetricReader::OnForceFlush(std::chrono::microseconds timeout) noexcept
 {
   std::unique_lock<std::mutex> lk_cv(force_flush_m_);
-  is_force_flush_pending_.store(true, std::memory_order_release);
-  auto break_condition = [this]() {
+  std::uint64_t current_sequence =
+      force_flush_pending_sequence_.fetch_add(1, std::memory_order_release) + 1;
+  auto break_condition = [this, current_sequence]() {
     if (IsShutdown())
     {
       return true;
     }
 
-    // Wake up the worker thread once.
-    if (is_force_flush_pending_.load(std::memory_order_acquire))
+    // Wake up the worker thread.
+    if (force_flush_pending_sequence_.load(std::memory_order_acquire) >
+        force_flush_notified_sequence_.load(std::memory_order_acquire))
     {
       is_force_wakeup_background_worker_.store(true, std::memory_order_release);
-      cv_.notify_one();
+      cv_.notify_all();
     }
-    return is_force_flush_notified_.load(std::memory_order_acquire);
+    return force_flush_notified_sequence_.load(std::memory_order_acquire) >= current_sequence;
   };
 
-  auto wait_timeout = opentelemetry::common::DurationUtil::AdjustWaitForTimeout(
-      timeout, std::chrono::microseconds::zero());
+  std::chrono::microseconds wait_timeout =
+      opentelemetry::common::DurationUtil::AdjustWaitForTimeout(timeout,
+                                                                std::chrono::microseconds::zero());
   std::chrono::steady_clock::duration timeout_steady =
       std::chrono::duration_cast<std::chrono::steady_clock::duration>(wait_timeout);
   if (timeout_steady <= std::chrono::steady_clock::duration::zero())
@@ -141,29 +146,19 @@ bool PeriodicExportingMetricReader::OnForceFlush(std::chrono::microseconds timeo
   bool result = false;
   while (!result && timeout_steady > std::chrono::steady_clock::duration::zero())
   {
-    // When is_force_flush_notified_.store(true) and force_flush_cv_.notify_all() is called
-    // between is_force_flush_pending_.load() and force_flush_cv_.wait(). We must not wait
-    // for ever
+    // When force_flush_notified_sequence_.compare_exchange_strong(...) and
+    // force_flush_cv_.notify_all() is called between force_flush_pending_sequence_.load(...) and
+    // force_flush_cv_.wait(). We must not wait for ever
     std::chrono::steady_clock::time_point start_timepoint = std::chrono::steady_clock::now();
-    result = force_flush_cv_.wait_for(lk_cv, export_interval_millis_, break_condition);
-    timeout_steady -= std::chrono::steady_clock::now() - start_timepoint;
-  }
 
-  // If it will be already signaled, we must wait until notified.
-  // We use a spin lock here
-  if (false == is_force_flush_pending_.exchange(false, std::memory_order_acq_rel))
-  {
-    for (int retry_waiting_times = 0;
-         false == is_force_flush_notified_.load(std::memory_order_acquire); ++retry_waiting_times)
+    wait_timeout = export_interval_millis_;
+    if (wait_timeout > timeout_steady)
     {
-      opentelemetry::common::SpinLockMutex::fast_yield();
-      if ((retry_waiting_times & 127) == 127)
-      {
-        std::this_thread::yield();
-      }
+      wait_timeout = std::chrono::duration_cast<std::chrono::microseconds>(timeout_steady);
     }
+    result = force_flush_cv_.wait_for(lk_cv, wait_timeout, break_condition);
+    timeout_steady -= std::chrono::steady_clock::now() - start_timepoint;
   }
-  is_force_flush_notified_.store(false, std::memory_order_release);
 
   if (result)
   {
@@ -186,18 +181,15 @@ bool PeriodicExportingMetricReader::OnForceFlush(std::chrono::microseconds timeo
       result = false;
     }
   }
-  return result;
+  return result &&
+         force_flush_notified_sequence_.load(std::memory_order_acquire) >= current_sequence;
 }
 
 bool PeriodicExportingMetricReader::OnShutDown(std::chrono::microseconds timeout) noexcept
 {
   if (worker_thread_.joinable())
   {
-    {
-      // ensure that `cv_` is awaiting, and the update doesn't get lost
-      std::unique_lock<std::mutex> lk(cv_m_);
-      cv_.notify_all();
-    }
+    cv_.notify_all();
     worker_thread_.join();
   }
   return exporter_->Shutdown(timeout);