Github user zsxwing commented on the pull request:
https://github.com/apache/spark/pull/887#issuecomment-44239047
> cachedPeers is the result of a future, so it's probably not possible that
it's partially constructed.
One thread runs the following code:
```
*r = ...
set cachedPeers to r
```
And another thread runs the following code:
```
if (read cachedPeers != null) {
assertTrue(read *cachedPeers == read *r) // JVM allows this assertion
fails if reading and writing do not have the happen-before relation
}
```
I added volatile to force a happen-before relation between writing
`cachedPeers` and reading `cachedPeers`.
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