Merge pull request oneapi-src#1518 from PietroGhg/pietro/coverity

[NATIVECPU] Address coverity warnings in threadpool implementation

Author: kbenzie

source/adapters/native_cpu/threadpool.hpp

@@ -11,9 +11,10 @@
 #include <atomic>
 #include <condition_variable>
 #include <cstdlib>
+#include <forward_list>
 #include <functional>
 #include <future>
-#include <iostream>
+#include <iterator>
 #include <mutex>
 #include <numeric>
 #include <queue>
@@ -30,18 +31,14 @@ namespace detail {
 class worker_thread {
 public:
   // Initializes state, but does not start the worker thread
-  worker_thread() noexcept : m_isRunning(false), m_numTasks(0) {}
-
-  // Creates and launches the worker thread
-  inline void start(size_t threadId) {
+  worker_thread(size_t threadId) noexcept
+      : m_threadId(threadId), m_isRunning(false), m_numTasks(0) {
     std::lock_guard<std::mutex> lock(m_workMutex);
     if (this->is_running()) {
       return;
     }
-    m_threadId = threadId;
     m_worker = std::thread([this]() {
       while (true) {
-        // pin the thread to the cpu
         std::unique_lock<std::mutex> lock(m_workMutex);
         // Wait until there's work available
         m_startWorkCondition.wait(
@@ -51,7 +48,7 @@ class worker_thread {
           break;
         }
         // Retrieve a task from the queue
-        auto task = m_tasks.front();
+        worker_task_t task = std::move(m_tasks.front());
         m_tasks.pop();
 
         // Not modifying internal state anymore, can release the mutex
@@ -63,7 +60,7 @@ class worker_thread {
       }
     });
 
-    m_isRunning = true;
+    m_isRunning.store(true, std::memory_order_release);
   }
 
   inline void schedule(const worker_task_t &task) {
@@ -79,16 +76,12 @@ class worker_thread {
   size_t num_pending_tasks() const noexcept {
     // m_numTasks is an atomic counter because we don't want to lock the mutex
     // here, num_pending_tasks is only used for heuristics
-    return m_numTasks.load();
+    return m_numTasks.load(std::memory_order_acquire);
   }
 
   // Waits for all tasks to finish and destroys the worker thread
   inline void stop() {
-    {
-      // Notify the worker thread to stop executing
-      std::lock_guard<std::mutex> lock(m_workMutex);
-      m_isRunning = false;
-    }
+    m_isRunning.store(false, std::memory_order_release);
     m_startWorkCondition.notify_all();
     if (m_worker.joinable()) {
       // Wait for the worker thread to finish handling the task queue
@@ -97,18 +90,21 @@ class worker_thread {
   }
 
   // Checks whether the thread pool is currently running threads
-  inline bool is_running() const noexcept { return m_isRunning; }
+  inline bool is_running() const noexcept {
+    return m_isRunning.load(std::memory_order_acquire);
+  }
 
 private:
   // Unique ID identifying the thread in the threadpool
-  size_t m_threadId;
+  const size_t m_threadId;
+
   std::thread m_worker;
 
   std::mutex m_workMutex;
 
   std::condition_variable m_startWorkCondition;
 
-  bool m_isRunning;
+  std::atomic<bool> m_isRunning;
 
   std::queue<worker_task_t> m_tasks;
 
@@ -121,47 +117,21 @@ class worker_thread {
 // parameters and futures.
 class simple_thread_pool {
 public:
-  simple_thread_pool(size_t numThreads = 0) noexcept : m_isRunning(false) {
-    this->resize(numThreads);
-    this->start();
-  }
-
-  ~simple_thread_pool() { this->stop(); }
-
-  // Creates and launches the worker threads
-  inline void start() {
-    if (this->is_running()) {
-      return;
-    }
-    size_t threadId = 0;
-    for (auto &t : m_workers) {
-      t.start(threadId);
-      threadId++;
+  simple_thread_pool() noexcept
+      : m_isRunning(false), m_numThreads(get_num_threads()) {
+    for (size_t i = 0; i < m_numThreads; i++) {
+      m_workers.emplace_front(i);
     }
     m_isRunning.store(true, std::memory_order_release);
   }
 
-  // Waits for all tasks to finish and destroys the worker threads
-  inline void stop() {
+  ~simple_thread_pool() {
     for (auto &t : m_workers) {
       t.stop();
     }
     m_isRunning.store(false, std::memory_order_release);
   }
 
-  inline void resize(size_t numThreads) {
-    char *envVar = std::getenv("SYCL_NATIVE_CPU_HOST_THREADS");
-    if (envVar) {
-      numThreads = std::stoul(envVar);
-    }
-    if (numThreads == 0) {
-      numThreads = std::thread::hardware_concurrency();
-    }
-    if (!this->is_running() && (numThreads != this->num_threads())) {
-      m_workers = decltype(m_workers)(numThreads);
-    }
-  }
-
   inline void schedule(const worker_task_t &task) {
     // Schedule the task on the best available worker thread
     this->best_worker().schedule(task);
@@ -171,7 +141,7 @@ class simple_thread_pool {
     return m_isRunning.load(std::memory_order_acquire);
   }
 
-  inline size_t num_threads() const noexcept { return m_workers.size(); }
+  inline size_t num_threads() const noexcept { return m_numThreads; }
 
   inline size_t num_pending_tasks() const noexcept {
     return std::accumulate(std::begin(m_workers), std::end(m_workers),
@@ -201,24 +171,32 @@ class simple_thread_pool {
   }
 
 private:
-  std::vector<worker_thread> m_workers;
+  static size_t get_num_threads() {
+    size_t numThreads;
+    char *envVar = std::getenv("SYCL_NATIVE_CPU_HOST_THREADS");
+    if (envVar) {
+      numThreads = std::stoul(envVar);
+    } else {
+      numThreads = std::thread::hardware_concurrency();
+    }
+    return numThreads;
+  }
+
+  std::forward_list<worker_thread> m_workers;
 
   std::atomic<bool> m_isRunning;
+
+  const size_t m_numThreads;
 };
 } // namespace detail
 
 template <typename ThreadPoolT> class threadpool_interface {
   ThreadPoolT threadpool;
 
 public:
-  void start() { threadpool.start(); }
-
-  void stop() { threadpool.stop(); }
-
   size_t num_threads() const noexcept { return threadpool.num_threads(); }
 
-  threadpool_interface(size_t numThreads) : threadpool(numThreads) {}
-  threadpool_interface() : threadpool(0) {}
+  threadpool_interface() : threadpool() {}
 
   auto schedule_task(worker_task_t &&task) {
     auto workerTask = std::make_shared<std::packaged_task<void(size_t)>>(