[HBASE-18628] ZKPermissionWatcher blocks all ZK notifications

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Fix Version/s: 1.4.0, 2.0.0-alpha-3, 2.0.0
• Component/s: regionserver
• Hadoop Flags: Reviewed
• Assignee: Mike Drob
• Reporter: Mike Drob
• Created: 18/Aug/17 14:36
• Updated: 01/Aug/18 06:22
• Resolved: 24/Aug/17 01:07

Description

Buckle up folks, we're going for a ride here. I've seeing this on a branch-2 based build, but I think the problem will affect branch-1 as well. I'm not able to easily reproduce the issue, but it will usually come up within an hour on a given cluster that I have, at which point the problem persists until an RS restart. I've been seeing the problem and paying attention for maybe two months, but I suspect it's been happening much longer than that.

Problem

When running in a secure cluster, sometimes the ZK EventThread will get stuck on a permissions update and not be able to process new notifications. This happens to also block flush and snapshot, which is how we found it.

Analysis

The main smoking gun is seeing this in repeated jstacks:

"main-EventThread" #43 daemon prio=5 os_prio=0 tid=0x00007f0b92644000 nid=0x6e69 waiting on condition [0x00007f0b6730f000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at org.apache.hadoop.hbase.security.access.ZKPermissionWatcher.nodeChildrenChanged(ZKPermissionWatcher.java:191)
        at org.apache.hadoop.hbase.zookeeper.ZooKeeperWatcher.process(ZooKeeperWatcher.java:503)
        at org.apache.zookeeper.ClientCnxn$EventThread.processEvent(ClientCnxn.java:522)
        at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:498)

That sleep is a 20ms sleep in an AtomicReference.compareAndSet loop - but it never gets past the condition.

        while (!nodes.compareAndSet(null, nodeList)) {
          try {
            Thread.sleep(20);
          } catch (InterruptedException e) {
            LOG.warn("Interrupted while setting node list", e);
            Thread.currentThread().interrupt();
          }
        }

The warning never shows up in the logs, it just keeps looping and looping. The last relevant line from the watcher in logs is:

2017-08-17 21:25:12,379 DEBUG org.apache.hadoop.hbase.zookeeper.ZooKeeperWatcher: regionserver:22101-0x15df38884c80024, quorum=zk1:2181,zk2:2181,zk3:2181, baseZNode=/hbase Received ZooKeeper Event, type=NodeChildrenChanged, state=SyncConnected, path=/hbase/acl

Which makes sense, because the code snippet is from permission watcher's nodeChildrenChanged handler.

The separate thread introduced in HBASE-14370 is present, but not doing anything. And this event hasn't gotten to the part where it splits off into a thread:

"zk-permission-watcher4-thread-1" #160 daemon prio=5 os_prio=0 tid=0x0000000001750800 nid=0x6fd9 waiting on condition [0x00007f0b5dce5000]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00000007436ecea0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
        at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
        at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)

Solutions

There's a few approaches we can take to fix this, I think they are all complimentary. It might be useful to file subtasks or new issues for some of the solutions if they are longer term.

1. Move flush and snapshot to ProcedureV2. This makes my proximate problem go away, but it's only relevant to branch-2 and master, and doesn't fix anything on branch-1. Also, Permissions updates would still get stuck, preventing future permissions updates. I think this is important long term for the robustness of the system, but not a viable short term fix.
2. Add an Executor to ZookeeperWatcher and launch threads from there. Maybe we'd want to pull the Executor out of ZKPW, but that's not strictly necessary and can be optimized later – if we're already threading, then adding another layer isn't a huge cost.
3. Figure out the race condition or logic problem that causes nodes to be non-null above. I've tried looking at this and visual inspection isn't getting me anywhere.

Issue Links

• is related to: HBASE-14370 Use separate thread for calling ZKPermissionWatcher#refreshNodes()