OpenCL User Events / Cant Release Objects? - opencl

Question

NOTE: Enqueued commands that specify user events in the
event_wait_list argument of clEnqueue*** commands must ensure that
the status of these user events being waited on are set using
clSetUserEventStatus before any OpenCL APIs that release OpenCL
objects except for event objects are called; otherwise the behavior
is undefined.
So if I have a user event being waited on in queue, I can't call release on any OpenCL object?
This seems like a strange requirement? What is the purpose of it? Or, why is it so?
The example they give is:
ev1 = clCreateUserEvent(ctx, NULL);
clEnqueueWriteBuffer(cq, buf1, CL_FALSE, ..., 1, &ev1, NULL);
clEnqueueWriteBuffer(cq, buf2, CL_FALSE,...);
clReleaseMemObject(buf2); // <--- UNDEFINED
clSetUserEventStatus(ev1, CL_COMPLETE);
Which causes undefined behaviour?

Answer 1

Consider the example that they give.
We have an in-order queue and we create a user event:
ev1 = clCreateUserEvent(ctx, NULL); // (1)
We then want to enqueue a write to a buffer, but we want it to wait for our event:
clEnqueueWriteBuffer(cq, buf1, CL_FALSE, ..., 1, &ev1, NULL); // (2)
We want to write another buffer after the buffer in the previous one (which is waiting on our event):
clEnqueueWriteBuffer(cq, buf2, CL_FALSE,...); // (3)
We release the buffer from the second clEnqueueWriteBuffer which hasn't gone through yet because we are still waiting for the user event. In this case (4) happens before (3), so we don't know what will happen as the memory object is freed.
clReleaseMemObject(buf2); // <--- UNDEFINED // (4)
We finally complete our user event which causes (2) and (3) to occur after (4) has already completed.
clSetUserEventStatus(ev1, CL_COMPLETE); // (5)
Basically, performing computation this way can cause problems because clReleaseMemObject doesn't get inserted into the clCommandQueue and can break the dependencies we expect.