[jira] [Updated] (ZOOKEEPER-3072) Race condition in throttling

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ZOOKEEPER-3072?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

ASF GitHub Bot updated ZOOKEEPER-3072:
--------------------------------------
    Labels: pull-request-available  (was: )

> Race condition in throttling
> ----------------------------
>
>                 Key: ZOOKEEPER-3072
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3072
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.5.0, 3.5.1, 3.5.2, 3.5.3, 3.5.4
>            Reporter: Botond Hejj
>            Priority: Major
>              Labels: pull-request-available
>
> There is a race condition in the server throttling code. It is possible that the disableRecv is called after enableRecv.
> Basically, the I/O work thread does this in processPacket: [https://github.com/apache/zookeeper/blob/release-3.5.3/src/java/main/org/apache/zookeeper/server/ZooKeeperServer.java#L1102] 
>                 submitRequest(si);
>             }
>         }
>         cnxn.incrOutstandingRequests(h);
>     }
>  
> incrOutstandingRequests() checks for limit breach, and potentially turns on throttling, [https://github.com/apache/zookeeper/blob/release-3.5.3/src/java/main/org/apache/zookeeper/server/NIOServerCnxn.java#L384]
>  
> submitRequest() will create a logical request and en-queue it so that Processor thread can pick it up. After being de-queued by Processor thread, it does necessary handling, and then calls this [https://github.com/apache/zookeeper/blob/release-3.5.3/src/java/main/org/apache/zookeeper/server/FinalRequestProcessor.java#L459] :
>  
>             cnxn.sendResponse(hdr, rsp, "response");
>  
> and in sendResponse(), it first appends to outgoing buffer, and then checks if un-throttle is needed:  [https://github.com/apache/zookeeper/blob/release-3.5.3/src/java/main/org/apache/zookeeper/server/NIOServerCnxn.java#L708]
>  
> However, if there is a context switch between submitRequest() and cnxn.incrOutstandingRequests(), so that Processor thread completes cnxn.sendResponse() call before I/O thread switches back, then enableRecv() will happen before disableRecv(), and enableRecv() will fail the CAS ops, while disableRecv() will succeed, resulting in a deadlock: un-throttle is needed for letting in requests, and sendResponse is needed to trigger un-throttle, but sendResponse() requires an incoming message. From that point on, ZK server will no longer select the affected client socket for read, leading to the observed client-side failure in the subject.
> If you would like to reproduce this than setting the globalOutstandingLimit down to 1 makes this reproducible easier as throttling starts with less requests. 
>  



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