a question about gdr_copy_from_mapping_internal's note.

[jams99]

gdr_copy_from_mapping_internal comment that:
// note: fencing is not needed because plain stores are used
// if non-temporal/uncached stores were used on x86, a proper fence would be needed instead

I think i can not understand the meaning, and i guess:
non-temporal store may out of order, so gdr_copy_from_mapping_internal need fence,
but, on x86, uncached memory access is strong order.

Can anybody help explain why uncached stores need fence?

Thanks.