MESI
With the introduction of multi-threading, multiple processes could run in parallel and interact with the same memory locations (obviously). As L-Cache is not the memory per se, but a replication, it can be set into an incoherent state.
MESI being the protocol that ensures the global correctness. There are four states that a cache line can have, they being Modified, Exclusive, Shared, and Invalid. The state "modified" is when the cache has been mutated, but changes not flushed towards memory, "exclusive" when only a core has the memory region in the cache, "shared" when multiple cores have a memory region in cache, and invalid when the state in the cache is not coherent in comparison with memory.
When an operation uses memory in the state "invalid", a cache miss is issued and the cache updated from the memory state. As for the other states, I did find reliable data in regards to the behavior. Thus I assume it's undocumented behavior.
This state mechanism is ensured by a core analysing other core's memory bus, therefore if a mutation is flushed into memory, the L-Cache of the other cores will be set invalid.
In conclusion, MESI ensures L-Cache is coherent to the memory state.