[SYCL] Add one more WA on Win for shutdown process (#19195)

WA for host task threads handling.
We must call join() or detach() on host task execution thread to avoid
UB.
DLLMain parameter lpReserved == NULL if library is unloaded via
FreeLibrary.
In this case we can't join threads within DllMain call due to global
loader lock
and DLL_THREAD_DETACH signalling.
lpReserved != NULL if library is unloaded during process termination. In
this case
Windows terminates threads but leave them in signalled state, prevents
DLL_THREAD_DETACH notification and we can call join() as NOP.

Note that FreeLibrary called with sycl RT handle does not guarantee
which path
will be used. Windows can (and actually does) simply postpone actual
unloading
till the end of program.

---------

Signed-off-by: Tikhomirova, Kseniya <[email protected]>

Author: KseniyaTikhomirova

sycl/source/detail/global_handler.cpp

@@ -38,7 +38,7 @@ using LockGuard = std::lock_guard<SpinLock>;
 SpinLock GlobalHandler::MSyclGlobalHandlerProtector{};
 
 // forward decl
-void shutdown_early();
+void shutdown_early(bool);
 void shutdown_late();
 #ifdef _WIN32
 BOOL isLinkedStatically();
@@ -287,12 +287,12 @@ struct StaticVarShutdownHandler {
       // If statically linked, DllMain will not be called. So we do its work
       // here.
       if (isLinkedStatically()) {
-        shutdown_early();
+        shutdown_early(true);
       }
 
       shutdown_late();
 #else
-      shutdown_early();
+      shutdown_early(true);
 #endif
     } catch (std::exception &e) {
       __SYCL_REPORT_EXCEPTION_TO_STREAM(
@@ -347,7 +347,10 @@ void GlobalHandler::drainThreadPool() {
     MHostTaskThreadPool.Inst->drain();
 }
 
-void shutdown_early() {
+// Note: this function can be called on Windows twice:
+//  1) when library is unloaded via FreeLibrary
+//  2) when process is being terminated
+void shutdown_early(bool CanJoinThreads = true) {
   const LockGuard Lock{GlobalHandler::MSyclGlobalHandlerProtector};
   GlobalHandler *&Handler = GlobalHandler::getInstancePtr();
   if (!Handler)
@@ -366,8 +369,10 @@ void shutdown_early() {
   // upon its release
   Handler->prepareSchedulerToRelease(true);
 
-  if (Handler->MHostTaskThreadPool.Inst)
-    Handler->MHostTaskThreadPool.Inst->finishAndWait();
+  if (Handler->MHostTaskThreadPool.Inst) {
+    Handler->MHostTaskThreadPool.Inst->finishAndWait(CanJoinThreads);
+    Handler->MHostTaskThreadPool.Inst.reset(nullptr);
+  }
 
   // This releases OUR reference to the default context, but
   // other may yet have refs
@@ -428,7 +433,14 @@ extern "C" __SYCL_EXPORT BOOL WINAPI DllMain(HINSTANCE hinstDLL,
       std::cout << "---> DLL_PROCESS_DETACH syclx.dll\n" << std::endl;
 
     try {
-      shutdown_early();
+      // WA for threads handling. We must call join() or detach() on host task
+      // execution thread to avoid UB. lpReserved == NULL if library is unloaded
+      // via FreeLibrary. In this case we can't join threads within DllMain call
+      // due to global loader lock and DLL_THREAD_DETACH signalling. lpReserved
+      // != NULL if library is unloaded during process termination. In this case
+      // Windows terminates threads but leave them in signalled state, prevents
+      // DLL_THREAD_DETACH notification and we can call join() as NOP.
+      shutdown_early(lpReserved != NULL);
     } catch (std::exception &e) {
       __SYCL_REPORT_EXCEPTION_TO_STREAM("exception in DLL_PROCESS_DETACH", e);
       return FALSE;

sycl/source/detail/global_handler.hpp

@@ -98,7 +98,7 @@ class GlobalHandler {
 
   bool OkToDefer = true;
 
-  friend void shutdown_early();
+  friend void shutdown_early(bool);
   friend void shutdown_late();
   friend class ObjectUsageCounter;
   static GlobalHandler *&getInstancePtr();

sycl/source/detail/thread_pool.hpp

@@ -32,15 +32,43 @@ class ThreadPool {
   bool MStop = false;
   std::atomic_uint MJobsInPool;
 
+#ifdef _WIN32
+  class ThreadExitTracker {
+  public:
+    void wait(size_t ThreadCount) {
+      std::unique_lock<std::mutex> lk(MWorkerExitMutex);
+      MWorkerExitCV.wait(
+          lk, [&ThreadCount, this] { return MWorkerExitCount == ThreadCount; });
+    }
+
+    void signalAboutExit() {
+      {
+        std::lock_guard<std::mutex> lk(MWorkerExitMutex);
+        MWorkerExitCount++;
+      }
+      MWorkerExitCV.notify_one();
+    }
+
+  private:
+    std::mutex MWorkerExitMutex;
+    std::condition_variable MWorkerExitCV;
+    size_t MWorkerExitCount{};
+  } WinThreadExitTracker;
+#endif
+
   void worker() {
     GlobalHandler::instance().registerSchedulerUsage(/*ModifyCounter*/ false);
     std::unique_lock<std::mutex> Lock(MJobQueueMutex);
     while (true) {
       MDoSmthOrStop.wait(Lock,
                          [this]() { return !MJobQueue.empty() || MStop; });
 
-      if (MStop)
-        break;
+      if (MStop) {
+#ifdef _WIN32
+        WinThreadExitTracker.signalAboutExit();
+#endif
+        return;
+      }
 
       std::function<void()> Job = std::move(MJobQueue.front());
       MJobQueue.pop();
@@ -76,21 +104,33 @@ class ThreadPool {
   ~ThreadPool() {
     try {
 #ifndef _WIN32
-      finishAndWait();
+      finishAndWait(true);
 #endif
     } catch (std::exception &e) {
       __SYCL_REPORT_EXCEPTION_TO_STREAM("exception in ~ThreadPool", e);
     }
   }
 
-  void finishAndWait() {
+  void finishAndWait(bool CanJoinThreads) {
     {
       std::lock_guard<std::mutex> Lock(MJobQueueMutex);
       MStop = true;
     }
 
     MDoSmthOrStop.notify_all();
 
+#ifdef _WIN32
+    if (!CanJoinThreads) {
+      WinThreadExitTracker.wait(MThreadCount);
+      for (std::thread &Thread : MLaunchedThreads)
+        Thread.detach();
+      return;
+    }
+#else
+    // We always can join on Linux.
+    std::ignore = CanJoinThreads;
+#endif
+
     for (std::thread &Thread : MLaunchedThreads)
       if (Thread.joinable())
         Thread.join();