Akira Ajisaka commented on HDFS-11180:

bq. NameNode holds a lock of FSEditLog and requires a lock of MetricsSystemImpl 
when registering IPCLoggerChannel metrics.
It looks like this deadlock can happen only when the QJM is used.
In addition, this deadlock can happen only when there is at least one metrics 
sink registered to MetricsSystem. If there is no metrics sink, 
MetricsSystemImpl.sampleMetrics can not be called.

> Intermittent deadlock in NameNode when failover happens.
> --------------------------------------------------------
>                 Key: HDFS-11180
>                 URL: https://issues.apache.org/jira/browse/HDFS-11180
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>    Affects Versions: 2.6.0
>            Reporter: Abhishek Modi
>            Assignee: Akira Ajisaka
>            Priority: Blocker
>              Labels: high-availability
>         Attachments: HDFS-11180.00.patch, HDFS-11180.01.patch, 
> HDFS-11180.02.patch, HDFS-11180.03.patch, HDFS-11180.04.patch, jstack.log
> It is happening due to metrics getting updated at the same time when failover 
> is happening. Please find attached jstack at that point of time.

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