[SYCL] Fix a thread pool data race during shutdown (#15535)

The flag that kills the threads waiting for work was an atomic bool, but
that is not enough: any modification of a condition must be made under
the mutex associated with that condition variable. Otherwise, the bool
value flip and the call to notify might happen after the condition check
in the other thread, but before it starts waiting for notifications.

sycl/source/detail/thread_pool.hpp

@@ -29,17 +29,17 @@ class ThreadPool {
   std::queue<std::function<void()>> MJobQueue;
   std::mutex MJobQueueMutex;
   std::condition_variable MDoSmthOrStop;
-  std::atomic_bool MStop;
+  bool MStop = false;
   std::atomic_uint MJobsInPool;
 
   void worker() {
     GlobalHandler::instance().registerSchedulerUsage(/*ModifyCounter*/ false);
     std::unique_lock<std::mutex> Lock(MJobQueueMutex);
     while (true) {
-      MDoSmthOrStop.wait(
-          Lock, [this]() { return !MJobQueue.empty() || MStop.load(); });
+      MDoSmthOrStop.wait(Lock,
+                         [this]() { return !MJobQueue.empty() || MStop; });
 
-      if (MStop.load())
+      if (MStop)
         break;
 
       std::function<void()> Job = std::move(MJobQueue.front());
@@ -57,7 +57,6 @@ class ThreadPool {
   void start() {
     MLaunchedThreads.reserve(MThreadCount);
 
-    MStop.store(false);
     MJobsInPool.store(0);
 
     for (size_t Idx = 0; Idx < MThreadCount; ++Idx)
@@ -83,7 +82,10 @@ class ThreadPool {
   }
 
   void finishAndWait() {
-    MStop.store(true);
+    {
+      std::lock_guard<std::mutex> Lock(MJobQueueMutex);
+      MStop = true;
+    }
 
     MDoSmthOrStop.notify_all();
 

sycl/test-e2e/Regression/unwaited_for_host_task.cpp

@@ -0,0 +1,16 @@
+// RUN: %{build} -o %t.out
+// RUN: %{run} %t.out
+
+#include <sycl/detail/core.hpp>
+
+using namespace sycl;
+
+// This test checks that host tasks that haven't been waited for do not cause
+// issues during shutdown.
+int main(int argc, char *argv[]) {
+  queue q;
+
+  q.submit([&](handler &h) { h.host_task([=]() {}); });
+
+  return 0;
+}