Atomicity

• Even with atomic bus the protocol requires multiple operations by multiple controllers, and multiple requests can be outstanding at once

• P1 wants to perform a BusRdX but cannot get access, meanwhile P2 is performing a BusRd on data P1 has modified

Consider two caches simultaneously issuing write to same block they hold shared

• P1 promotes S-->M and issues upgrade
• P2 does too, but wins arbitration
• P1 downgrades M--> I, but upgrade request till out
• P1’s revises to be BusRdX
• Therefore, snoop against requests

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