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  1. JDK
  2. JDK-6261344

(se) Select may block forever on Windows even if it should have returned immediately

    Details

    • Subcomponent:
    • Resolved In Build:
      b59
    • CPU:
      x86
    • OS:
      windows_xp

      Description

      FULL PRODUCT VERSION :
      Observed on 1.5.0_02 and verified in java 1.5 jdk windows specific source code

      ADDITIONAL OS VERSION INFORMATION :
      Any version of Windows

      A DESCRIPTION OF THE PROBLEM :
      There is a racing condition in sun.nio.ch.WindowsSelectorImpl. The racing condition may make a call to java.nio.channels.Selector.select block forever even if it was supposed to return immediately.

      Consider the case where a thread A calls doSelect. doSelect will call adjustThreadsCount. Suppose adjustThreadsCount creates a new SelectThread B. Suppose A is then suspended for some time and that instead B starts executing. From SelectThread.run B will then make a call to startLock.waitForStart and then to subSelector.poll. Suppose this call returns immediately, then B will continue with calling finishLock.threadsFinished and then startLock.waitForStart which will then block. Suppose that A is not resumed until this point. Then A will call finishLock.reset() and this will prevent A from ever returning from doSelect.

      The bug I repported is due to a programming error in sun.nio.ch.WindowsSelectorImpl, not due to limitations on the Windows platform. I found the bug as follows: From time to time my program blocked completely. I then reviewed the jdk source code where I found the bug I reported here. This bug explains the behaviour of my program. Also, during debugging I wrote out thread dumps and these dumps confirm that the reported bug is indeed a bug. Finally, the problem happens exactly when the number of sockets in the selector goes from 1023 to 1024. These numbers are magic in sun.nio.ch.WindowsSelectorImpl, not in my program. So I am almost as sure as you can get that this is indeed a bug in the jdk.

      REPRODUCIBILITY :
      This bug can be reproduced occasionally.

      CUSTOMER SUBMITTED WORKAROUND :
      Never register use more than 1023 keys on a selector.
      ###@###.### 2005-04-26 11:59:55 GMT

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            • Assignee:
              sherman Xueming Shen
              Reporter:
              ndcosta Nelson Dcosta (Inactive)
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