Ownership in this title means exclusive write permission: the M state, granted by H.
A store to one field does not replace every byte in its 64-byte line. A write-allocate miss therefore needs both exclusive permission and the current line contents. The traces show one value X, but Data carries the whole line.
Two basic cases differ only in what H must do before it can grant M: nothing, or invalidate the readers. A third case, where another cache holds the line dirty, is the exclusive-permission transfer of Three-Hop Versus Four-Hop.
With H showing U, the write miss is the cold write traced in SWMR Precisely: 1. GetM, H reads memory, 2. Data(X=0, M). C0 waits in IM^D ('was I, wants M, waiting for Data'), cannot load or store, and enters M when Data arrives. Two legs, no other cache involved.
Trace: The Course Model in Action, steps 3 to 6 and L (MSI). Same roles: from the checkpoint C0 S / 0, C1 I / -, H S{C0}, memory = 0, C1 wants to store 1. C1 has no copy, so it needs the value as well as M, and C0's read copy must go before C1 may write. C1 sends GetM and waits in IM^D, H sends Inv to C0, C0 answers Inv-Ack to H, H reads memory = 0 and grants Data(X=0, M), then C1 stores 1.
After the Inv-Ack the sharer set is empty, but H is still busy with C1's GetM, so it is not free for another request. Memory is current throughout, because S copies are clean in MSI.
Part 1 draws the memory read after the Inv-Ack, but nothing forces that order. H may start the memory read as soon as the GetM arrives and overlap it with the Inv round trip. That hides the memory latency, not the ack round trip: H must not send Data before the Inv-Ack, because with no FIFO order C1 could store while C0 still reads 0.
Compare the two cases: from U the miss costs two legs, from S{C0} four, and the extra two are the Inv round trip. The value comes from memory in both, because no cache holds it dirty. If C0 held M, H would forward instead, as the three-hop lesson shows.
C1 has no copy, so it needs the line's bytes as well as permission. In the upgrade the requester already holds a valid S copy, so H sends permission only.