CapMoon opened a new pull request, #8677:
URL: https://github.com/apache/hadoop/pull/8677
HDFS-17962. NNTop may lose user metrics during expired window cleanup
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### Description of PR
NNTop stores a RollingWindow for each (metric, user) pair in a
ConcurrentHashMap.
In RollingWindowManager#getTopUsersForMetric(), expired user windows are
removed when getSum(time) returns 0. Previously, the cleanup used an iterator
to remove the map entry, while RollingWindowManager#recordMetric() obtained a
RollingWindow from the map and incremented it outside of any per-key atomic
operation.
This allows the following race:
1. A recordMetric() thread obtains an existing RollingWindow for a user.
2. A snapshot thread observes that the window sum is 0 and removes the
user-to-window mapping.
3. The recordMetric() thread increments the RollingWindow it obtained
earlier.
4. The increment is applied to an object that is no longer in the map, so
the metric is lost from subsequent NNTop snapshots.
### How was this patch tested?
Tested by newly added unit test
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