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Date: Fri, 27 Nov 2009 11:37:57 -0500
Message-ID: <9ac0c6aa0911270837s6ab343an9fb6386cea4dcd43@mail.gmail.com>
Subject: deadlock in TestCrash
From: Michael McCandless <lucene@mikemccandless.com>
To: java-dev@lucene.apache.org
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RAMFile has this method:

  final synchronized byte[] addBuffer(int size) {
    byte[] buffer = newBuffer(size);
    if (directory!=null)
      synchronized (directory) {             // Ensure addition of
buffer and adjustment to directory size are atomic wrt directory
        buffers.add(buffer);
        directory.sizeInBytes += size;
        sizeInBytes += size;
      }
    else
      buffers.add(buffer);
    return buffer;
  }

But in working on LUCENE-2095, I'm seeing a deadlock, because that
method first syncs on RAMFile and then tries to sync on directory,
while the MockRAMDirectory.crash method, and I think maybe other
places, do the reverse.  I remember also hitting a different deadlock
from this in the past, and working around it.

I'd like to break the deadlock by changing RAMFile to this:

  final byte[] addBuffer(int size) {
    byte[] buffer = newBuffer(size);
    synchronized(this) {
      buffers.add(buffer);
      sizeInBytes += size;
    }

    if (directory != null) {
      synchronized (directory) {
        directory.sizeInBytes += size;
      }
    }
    return buffer;
  }

But, then, the addition of the buffer and the change of sizeInBytes is
no longer atomic wrt the directory (as the comment says).

So my question is... is this change OK?  Why is/was it so crucial that
the sizeInBytes & RAMFile's buffers really change only atomically?
Ie, it seems harmless if RAMDirectory.sizeInBytes() might not include
a buffer that's in the process of being added...?

(I do see TestRAMDirectory.testRAMDirectorySize requires this
atomicity, but, I can fix the test to only check the size in the
end...).

Mike

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