Github user vanzin commented on the issue:
https://github.com/apache/spark/pull/17610
> An AtomicBoolean will introduce a new lock
I don't think it introduces a new lock, just an easy way to make `stop()`
idempotent.
I guess the question to ask is (and is basically what you already asked):
does `stop()` need to hold any locks? That `synchronized` has been there since
the class was added, so it's hard to measure the original intent, but a quick
look at the code seems to show that a lock is not necessary, and a boolean to
check whether stop has already been called should be enough.
The new lock being added here only prevents two thread from concurrently
executing `stop()`. I think that's not something that usually happens, and if
it ever does, having one of them return immediately (instead of blocking)
shouldn't cause any problems.
But, at the end, either solution should be fine, as long as `stop()`
behaves correctly when it's not holding the "this" lock.
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