Request for review: Race conditions in java.nio.charset.Charset (original) (raw)

David Holmes - Sun Microsystems David.Holmes at Sun.COM
Thu Oct 8 10:59:54 UTC 2009

Hi Ulf,

Ulf Zibis said the following on 10/08/09 20:07:

Am 08.10.2009 06:35, David Holmes - Sun Microsystems schrieb:

Ulf Zibis said the following on 10/08/09 08:58:

For my better understanding: Can you explain me the real bug in http://bugs.sun.com/bugdatabase/viewbug.do?bugid=6881442. In my understanding, loading the "name" field twice is too only a performance bug. Please correct me!

Class.getName() could in theory return null and this is not allowed, hence this was a functional correctness issue. Hm, my problem is, I can't see any condition, how Class.getName() could return null, except native method getName0() would return null. But if native getName0() could return null, I can't see how the new code would prevent method Class.getName() from returning null.

It's a memory model issue. The code is like this:

  public String getName() {
      if (name == null)
         name = getName0();
      return name;
  }

but in theory, accoridng to the JMM experts, it could act as if it does this:

public String getName() { String tmp1 = name; // sees null String tmp2 = name; // sees non-null if (tmp2 == null) tmp1 = name = getName0(); return tmp1; }

imagine the temporaries are registers.

David

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