Fix race in events scenarios.

- It could happen that 2 threads try to update the same event within tree
- First thread starts to submit command, it releases cmdToSubmit
- Second thread doesn't see the command and follows
- Second thread thinks that it submitted the command so it follows to
subsequent steps which are child events notification
- We end up with corrupted enqueue sequence as child may submit prior to
parent.
- With this change each submit step is synchronized basing on task count
- When second thread enters submit command without task count being set
it wait for first thread to properly set it.

Change-Id: Ic2ddaea17f9af8cab6781320edae2c268dd0b189
Signed-off-by: Mrozek, Michal <michal.mrozek@intel.com>

unit_tests/event/user_events_tests_mt.cpp

@@ -120,6 +120,7 @@ TEST_F(EventTests, givenoneThreadUpdatingUserEventAnotherWaitingOnFinishWhenFini
         cl_event returnedEvent = nullptr;
 
         std::atomic_bool go{false};
+        std::atomic_bool updateEvent{true};
 
         std::thread t([&]() {
             while (!go)
@@ -132,7 +133,9 @@ TEST_F(EventTests, givenoneThreadUpdatingUserEventAnotherWaitingOnFinishWhenFini
         EXPECT_EQ(CL_SUCCESS, retVal);
 
         std::thread t2([&]() {
-            castToObject<Event>(returnedEvent)->updateExecutionStatus();
+            while (updateEvent) {
+                castToObject<Event>(returnedEvent)->updateExecutionStatus();
+            }
         });
 
         go = true;
@@ -141,6 +144,7 @@ TEST_F(EventTests, givenoneThreadUpdatingUserEventAnotherWaitingOnFinishWhenFini
         EXPECT_EQ(pCmdQ->latestTaskCountWaited, i + 1);
 
         t.join();
+        updateEvent = false;
         t2.join();
         clReleaseEvent(returnedEvent);
     }