[fpc-devel] TAVLTree(avl_tree.pp) thread safety, fcl-xml(DOM) is also concerned.

Inoussa OUEDRAOGO inoussa12 at gmail.com
Wed Aug 6 22:19:13 CEST 2008

2008/8/6 Mattias Gaertner <nc-gaertnma at netcologne.de>:
> On Wed, 6 Aug 2008 19:41:27 +0100
> "Inoussa OUEDRAOGO" <inoussa12 at gmail.com> wrote:
>> Hi,
>> TAVLTree in avl_tree.pp is not thread safe due to the node
>> allocation and de-allocation done through the global
>> declared "NodeMemManager" variable. TAVLTreeNodeMemManager
>> implementation is cleary not thread safe, which btw IMHO
>> is a good thing ( for performance reason).
>> Proposition :
>> (a) TAVLTree should allow, at construct time, to
>>     specify a Node memory manager which will be kept and used.
>>     If not specified the global one will be used.
>> (b) "NodeMemManager" should be declared as "ThreadVar".
>> This changes does not break the API while making the
>> implementation thread safe.
>> By the way note that the XML DOM's implementation
>> ( TDOMNode_WithChildren ) uses TAVLTree, so every code that uses the
>> fcl-xml package mainly the DOM unit, is not thread safe. If this
>> proposition is accepted it will be nice to have it introduced in the
>> soon to be release fpc 2.2.2, in the case that is still possible,
>> mainly for server programming.
>> Attached is a patch that demonstrate the above proposition.
> Providing a local node mem manager does not repair xml dom.

A "threadvar" declared variable is local to the calling thread.
But if object is modified in more than one thread, indeed there are
still some problems ( for objects using the default manager ) :
 Thread A create a node n1 and the node is released in ThreadB .

> Maybe move NodeMemManager to the interface, so that a user can replace
> it with a threadsafe one?

Good solution, but : if ThreadA create an avl object that is _exclusively_ used
in ThreadA, why should ThreadA wait in a multithreaded node mem manager?
That is when comes the constructor that takes a node mem manager.
That way, ThreadA could provide a local node mem manager that is not
multithreaded and thus not paying penalty of multithreaded node mem manager.

Inoussa O.

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