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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2711) Deadlock between concurrent 4LW commands that iterate over connections with Netty server
Date Fri, 10 Mar 2017 17:21:04 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2711?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15905432#comment-15905432
] 

ASF GitHub Bot commented on ZOOKEEPER-2711:
-------------------------------------------

Github user joshelser commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/186#discussion_r105444315
  
    --- Diff: src/java/main/org/apache/zookeeper/server/NettyServerCnxnFactory.java ---
    @@ -162,7 +162,7 @@ public void messageReceived(ChannelHandlerContext ctx, MessageEvent
e)
                                 + " from " + ctx.getChannel());
                     }
                     NettyServerCnxn cnxn = (NettyServerCnxn)ctx.getAttachment();
    -                synchronized(cnxn) {
    +                synchronized(cnxn.getRpcLock()) {
                         processMessage(e, cnxn);
    --- End diff --
    
    > I think I like your way better. Although it may be a good idea to have a comment
next to RPC_LOCK pointing to the JIRA
    
    Totally agree there :) 


> Deadlock between concurrent 4LW commands that iterate over connections with Netty server
> ----------------------------------------------------------------------------------------
>
>                 Key: ZOOKEEPER-2711
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2711
>             Project: ZooKeeper
>          Issue Type: Bug
>            Reporter: Josh Elser
>            Priority: Critical
>
> Observed the following issue in some $dayjob testing environments. Line numbers are a
little off compared to master/branch-3.5, but I did confirm the same issue exists there.
> With the NettyServerCnxnFactory, before a request is dispatched, the code synchronizes
on the {{NettyServerCnxn}} object. However, with some 4LW commands (like {{stat}}), each {{ServerCnxn}}
object is also synchronized to (safely) iterate over the internal contents of the object to
generate the necessary debug message. As such, multiple concurrent {{stat}} commands can both
lock their own {{NettyServerCnxn}} objects, and then be blocked waiting to lock each others'
{{ServerCnxn}} in the {{StatCommand}}, deadlocked.
> {noformat}
> "New I/O worker #55":
> 	at org.apache.zookeeper.server.ServerCnxn.dumpConnectionInfo(ServerCnxn.java:407)
> 	- waiting to lock <0x00000000fabc01b8> (a org.apache.zookeeper.server.NettyServerCnxn)
> 	at org.apache.zookeeper.server.NettyServerCnxn$StatCommand.commandRun(NettyServerCnxn.java:478)
> 	at org.apache.zookeeper.server.NettyServerCnxn$CommandThread.run(NettyServerCnxn.java:311)
> 	at org.apache.zookeeper.server.NettyServerCnxn$CommandThread.start(NettyServerCnxn.java:306)
> 	at org.apache.zookeeper.server.NettyServerCnxn.checkFourLetterWord(NettyServerCnxn.java:677)
> 	at org.apache.zookeeper.server.NettyServerCnxn.receiveMessage(NettyServerCnxn.java:790)
> 	at org.apache.zookeeper.server.NettyServerCnxnFactory$CnxnChannelHandler.processMessage(NettyServerCnxnFactory.java:211)
> 	at org.apache.zookeeper.server.NettyServerCnxnFactory$CnxnChannelHandler.messageReceived(NettyServerCnxnFactory.java:135)
> 	- locked <0x00000000fab68178> (a org.apache.zookeeper.server.NettyServerCnxn)
> 	at org.jboss.netty.channel.SimpleChannelHandler.handleUpstream(SimpleChannelHandler.java:88)
> 	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564)
> 	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559)
> 	at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:268)
> 	at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:255)
> 	at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:88)
> 	at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:109)
> 	at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312)
> 	at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:90)
> 	at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)
> 	at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
> 	at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42)
> 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
> 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
> 	at java.lang.Thread.run(Thread.java:745)
> "New I/O worker #51":
> 	at org.apache.zookeeper.server.ServerCnxn.dumpConnectionInfo(ServerCnxn.java:407)
> 	- waiting to lock <0x00000000fab68178> (a org.apache.zookeeper.server.NettyServerCnxn)
> 	at org.apache.zookeeper.server.NettyServerCnxn$StatCommand.commandRun(NettyServerCnxn.java:478)
> 	at org.apache.zookeeper.server.NettyServerCnxn$CommandThread.run(NettyServerCnxn.java:311)
> 	at org.apache.zookeeper.server.NettyServerCnxn$CommandThread.start(NettyServerCnxn.java:306)
> 	at org.apache.zookeeper.server.NettyServerCnxn.checkFourLetterWord(NettyServerCnxn.java:677)
> 	at org.apache.zookeeper.server.NettyServerCnxn.receiveMessage(NettyServerCnxn.java:790)
> 	at org.apache.zookeeper.server.NettyServerCnxnFactory$CnxnChannelHandler.processMessage(NettyServerCnxnFactory.java:211)
> 	at org.apache.zookeeper.server.NettyServerCnxnFactory$CnxnChannelHandler.messageReceived(NettyServerCnxnFactory.java:135)
> 	- locked <0x00000000fabc01b8> (a org.apache.zookeeper.server.NettyServerCnxn)
> 	at org.jboss.netty.channel.SimpleChannelHandler.handleUpstream(SimpleChannelHandler.java:88)
> 	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564)
> 	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559)
> 	at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:268)
> 	at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:255)
> 	at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:88)
> 	at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:109)
> 	at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312)
> 	at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:90)
> 	at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)
> 	at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
> 	at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42)
> 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
> 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
> 	at java.lang.Thread.run(Thread.java:745)
> {noformat}
> It would appear that the synchronization on the {{NettyServerCnxn}} in {{NettyServerCnxnFactory}}
is to blame (and I can see why it was done originally). I think we can just use a different
Object (and monitor) to provide mutual exclusion at Netty layer (and avoid synchronization
issues at the "application" layer).



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