[fdo-internals] FOR REVIEW: RFC 47 - Add Thread Support to FdoIDisposable AddRef/Release

[Greg Boone]

Hi All,

I have written and posted FDO RFC 47: Add Thread Support to FdoIDisposable AddRef/Release, accessible at:

http://trac.osgeo.org/fdo/wiki/FDORfc47

Overview:

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.

Proposed Solution:

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

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.


Please refer to the RFC for a full description on the change and a working draft of the proposed solution.

All comments are welcome.

Regards,
Greg
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