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<p class=MsoNormal>Hi All,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I have written and posted FDO RFC 47: <b>Add</b> <b>Thread
Support to FdoIDisposable AddRef/Release</b>, accessible at:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:.5in'><a
href="http://trac.osgeo.org/fdo/wiki/FDORfc47">http://trac.osgeo.org/fdo/wiki/FDORfc47</a><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><b>Overview:<o:p></o:p></b></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='margin-left:.5in'><span style='color:black'>The
AddRef and Release methods on FdoIDisposable are not currently thread-safe. As
a consequence the m_refCount data member can get corrupted if two separate threads
simultaneously call any of these methods on the same object. This is known to
happen during garbage collection of Managed FDO objects. Because the managed
objects implement a finalizer, they are processed by a separate GC finalizer
thread. So while the GC finalizer thread is releasing ref counts on unmanaged
objects, the main thread can be simultaneously modifying ref-counts on those
same objects.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal><b><span style='color:black;letter-spacing:-.2pt'>Proposed
Solution:<o:p></o:p></span></b></p>
<p class=MsoNormal><span style='color:black;letter-spacing:-.2pt'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:.5in'><span style='color:black'>In
MapGuide the refcount threading issue is addressed by having the refcounted
classes extend from MgGuardDisposable, which in turn uses a mutex to limit
access to the refcount member variable to one thread at a time. A similar
solution will be implemented for the FDO API, with a slight variation. On
Linux, a mutex will be used. On Windows, the FDO API will use the
InterlockedIncrement and InterlockedDecrement methods<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:.5in'><span style='color:black'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:.5in'><span style='color:black'>On
Windows, the use of thread safe functions will always be activated in case
where unmanaged FDO objects are wrapped by a Managed object wrapper. Otherwise,
API callers will have the option of setting a per-object or global flag that
will activate thread-safe access of the reference count member variable.</span><span
style='color:black'><o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Please refer to the RFC for a full description on the change
and a working draft of the proposed solution. <o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>All comments are welcome.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Regards,<o:p></o:p></p>
<p class=MsoNormal>Greg<o:p></o:p></p>
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