[SYCL] Fix circular reference between events and queues (#1226)

There is a potential circular reference between queues and events as a queue can hold a collection of shared pointers to events and events may have a shared pointer to a queue. If a circular reference is created, the only way to currently break the circle is to call wait on the queue, which is not ensured to happen.

To mitigate this, this PR changes makes the reference from an event to a queue a weak pointer. Effectively this will mean that if all remaining references to a queue are from events the queue will die. This affects only an events ability to throw asynchronous errors through the async_handler of the queue if it is destroyed prematurely.

Signed-off-by: Steffen Larsen <[email protected]>

Author: steffenlarsen

sycl/source/detail/event_impl.cpp

@@ -114,8 +114,9 @@ void event_impl::wait_and_throw(
     if (Cmd)
       Cmd->getQueue()->throw_asynchronous();
   }
-  if (MQueue)
-    MQueue->throw_asynchronous();
+  QueueImplPtr Queue = MQueue.lock();
+  if (Queue)
+    Queue->throw_asynchronous();
 }
 
 template <>

sycl/source/detail/event_impl.hpp

@@ -25,6 +25,7 @@ class context_impl;
 using ContextImplPtr = std::shared_ptr<cl::sycl::detail::context_impl>;
 class queue_impl;
 using QueueImplPtr = std::shared_ptr<cl::sycl::detail::queue_impl>;
+using QueueImplWPtr = std::weak_ptr<cl::sycl::detail::queue_impl>;
 
 class event_impl {
 public:
@@ -147,7 +148,7 @@ class event_impl {
 private:
   RT::PiEvent MEvent = nullptr;
   ContextImplPtr MContext;
-  QueueImplPtr MQueue;
+  QueueImplWPtr MQueue;
   bool MOpenCLInterop = false;
   bool MHostEvent = true;
   std::unique_ptr<HostProfilingInfo> MHostProfilingInfo;