[HBASE-7160] Improve IdLock and remove its minor defects

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Won't Fix
• Assignee: Unassigned
• Reporter: Hiroshi Ikeda
• Created: 14/Nov/12 08:55
• Updated: 15/Jun/22 00:19
• Resolved: 15/Jun/22 00:19

Description

Combination of synchronizations and concurrent collections complicates the code, and it is hard to trace the code and to confirm its correctness. We should re-create the class and make it more understandable.

In the current code, I find the following minor defects:

(1) In the case that there is a waiting thread for a lock in getLockEntry() and another thread is releasing the lock by calling releaseLockEntry(), trying to get the lock with a 3rd thread by calling getLockEntry() falls into a busy loop until the waiting thread wakes up and gets the lock.

Even if notify() wakes up the blocked thread and causes a context switch to the waked thread immediately, synchronization might block the waked thread and cause another context switch, and the busy loop might continue for a while.

(2) In the same case as (1), since releasing the lock is merely notifying without removing the lock-entry from the map, interrupting the waiting thread might leave an unused lock-entry (entry.numWaiters == 0) in the map. This is a memory leak unless the id of the lock is used again.

Issue Links

• is related to: HBASE-5898 Consider double-checked locking for block cache lock