Address feedback

Rename counter
Add more comments describing the blocking behavior
Add comments for potential deadlock scenario

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventPipeEventProvider.cs

@@ -85,6 +85,9 @@ internal override unsafe void Register(EventSource eventSource)
         }
 
         // Unregister an event provider.
+        // Calling Unregister within a Callback will result in a deadlock
+        // as deleting the provider with an active tracing session will block
+        // until all of the provider's callbacks are completed.
         internal override void Unregister()
         {
             if (_provHandle != 0)

src/native/eventpipe/ep-provider.c

@@ -81,7 +81,7 @@ provider_prepare_callback_data (
 	ep_requires_lock_held ();
 
 	if (provider->callback_func != NULL)
-		provider->uninvoked_prepared_callbacks_counter++;
+		provider->callbacks_pending++;
 
 	return ep_provider_callback_data_init (
 		provider_callback_data,
@@ -209,7 +209,7 @@ ep_provider_alloc (
 	instance->config = config;
 	instance->delete_deferred = false;
 	instance->sessions = 0;
-	instance->uninvoked_prepared_callbacks_counter = 0;
+	instance->callbacks_pending = 0;
 
 ep_on_exit:
 	return instance;
@@ -444,8 +444,8 @@ provider_invoke_callback (EventPipeProviderCallbackData *provider_callback_data)
 	EP_LOCK_ENTER (section1)
 		if (callback_function != NULL) {
 			EventPipeProvider *provider = provider_callback_data->provider;
-			provider->uninvoked_prepared_callbacks_counter--;
-			if (provider->uninvoked_prepared_callbacks_counter == 0 && provider->callback_func == NULL) {
+			provider->callbacks_pending--;
+			if (provider->callbacks_pending == 0 && provider->callback_func == NULL) {
 				// ep_delete_provider deferred provider deletion and is waiting for all in-flight callbacks
 				// to complete. This is the last callback, so signal completion.
 				ep_rt_wait_event_set (&provider->callbacks_complete_event);

src/native/eventpipe/ep-provider.h

@@ -42,10 +42,9 @@ struct _EventPipeProvider_Internal {
 	// True if the provider has been deleted, but that deletion
 	// has been deferred until tracing is stopped.
 	bool delete_deferred;
-	// The number of EventPipeProvider callbacks that have been
-	// prepared but not yet invoked.
-	// Used to determine when it is safe to delete a provider.
-	int64_t uninvoked_prepared_callbacks_counter;
+	// The number of pending EventPipeProvider callbacks that have
+	// not completed.
+	int64_t callbacks_pending;
 	// Event object used to signal eventpipe provider deletion
 	// that all in flight callbacks have completed.
 	ep_rt_wait_event_handle_t callbacks_complete_event;

src/native/eventpipe/ep.c

@@ -1323,16 +1323,24 @@ ep_delete_provider (EventPipeProvider *provider)
 			// Save the provider until the end of the tracing session.
 			ep_provider_set_delete_deferred (provider, true);
 
-			// the callback func must be previously set to null,
-			// otherwise callbacks might never stop coming
+			// The callback func must be previously set to null,
+			// otherwise callbacks might never stop coming.
 			EP_ASSERT (provider->callback_func == NULL);
-			if (provider->uninvoked_prepared_callbacks_counter > 0)
+
+			// Calling ep_delete_provider within a Callback will result in a deadlock
+			// as deleting the provider with an active tracing session will block
+			// until all of the provider's callbacks are completed.
+			if (provider->callbacks_pending > 0)
 				wait_for_provider_callbacks_completion = true;
 		} else {
 			config_delete_provider (ep_config_get (), provider);
 		}
 	EP_LOCK_EXIT (section1)
 
+	// Block provider deletion until all pending callbacks are completed.
+	// Helps prevent the EventPipeEventProvider Unregister logic from
+	// freeing freeing the provider's weak reference gchandle before
+	// callbacks using that handle have completed.
 	if (wait_for_provider_callbacks_completion)
 		ep_rt_wait_event_wait (&provider->callbacks_complete_event, EP_INFINITE_WAIT, false);