[YARN-4416] Deadlock due to synchronised get Methods in AbstractCSQueue

• Type: Sub-task
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.7.1
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Component/s: capacity scheduler, resourcemanager
• Assignee: Naganarasimha G R
• Reporter: Naganarasimha G R
• Created: 03/Dec/15 19:09
• Updated: 30/Aug/16 01:13
• Resolved: 17/Dec/15 19:25

Description

While debugging in eclipse came across a scenario where in i had to get to know the name of the queue but every time i tried to see the queue it was getting hung. On seeing the stack realized there was a deadlock but on analysis found out that it was only due to queue.toString() during debugging as AbstractCSQueue.getAbsoluteUsedCapacity was synchronized.
Hence we need to ensure following :

1. queueCapacity, resource-usage has their own read/write lock hence synchronization is not req
2. numContainers is volatile hence synchronization is not req.
3. read/write lock could be added to Ordering Policy. Read operations don't need synchronized. So getNumApplications doesn't need synchronized.
   (First 2 will be handled in this jira and the third will be handled in YARN-4443)