I have a (draft) PR written up that adds this error condition in #979, but I'm starting to have second thoughts whether it is actually possible to reliably return an error in this case. Consider a tricky case like the following (pseudocode):

event1 = clCreateUserEvent();
clEnqueueSVMUnmap(queue1, ptr, depends on event1);

event0 = clCreateUserEvent();
clEnqueueSVMMap(queue0, ptr, depends on event0);

Observations are:

We can't determine at the call to clEnqueueSVMUnmap whether the ptr is mapped or not - if event0 is set to complete first then the map will execute first, and if event1 is set to complete first then the unmap will execute first.
There are other cases without using user events where an SVM map may execute before an SVM unmap but where there are no guarantees.
Buffer and image memory objects don't have a similar problem for two reasons: the pointer to unmap is (usually) unknown until the memory object is mapped, therefore as a practical matter the call to map the memory object must (usually) occur before the call to unmap the memory object. It is also possible to query the memory object's map count, which provides another mechanism to determine if the memory object has been mapped or not.

Given (3), is the expectation that implementations track which SVM regions have been mapped similarly, even though there is (currently) no queryable map count? Should the example above generate an error because clEnqueueSVMUnmap was called before clEnqueueSVMMap, even though the map may actually execute before the unmap?

clEnqueueSVMUnmap #890

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