This is an accompanying blog for the www.1024cores.net site about lockfree/waitfree synchronization algorithms and data structures, scalability-oriented architecture, multicore design patterns, high-performance computing, threading technologies and libraries, message-passing systems and related topics.
Friday, January 7, 2011
Differential Reference Counting
Just published an article on Differential Reference Counting - reference counting algorithm which provides strong thread-safety and permits useful use cases that are not permitted by plain atomic reference counting (like boost::shared_ptr).
I guess this is not correct way to use the smart pointer. This does not differ from the variant with a regular global pointer, without any reference counting at all.
T * ptr;
// thread 1 ptr->SomeMethod(); //The problem is the same - who is freeing it at this time?
To use it properly, we have to increment the reference counter safely in the thread, who is already an owner of the resource :
// thread 1 shared_ptr ptr(new T); // now this thread is a sole owner of the resource
// lets share it with another thread. shared_ptr ptr2 = ptr;// The counter was incremented in this thread - nobody could free it.
CreateThreadWithContext(Thread2Func, ptr.get());
// thread 2 void Thread2Func(T * p) { shared_ptr ptr(p, false);// false means NOT to increment reference counter again
ptr->SomeMethod();// You can use it safely, You are already an owner, nobody can free it.
}// destructor of shared_ptr decrement reference counter, and free the resource, if this thread is its last owner.
It may be enough for safe life-time management.
But your idea is interesting as a access point to some resource.
> I guess this is not correct way to use the smart pointer.
It's an incorrect way to use a reference counting algorithm, if it does not support such usage. However, if it does support such usage, then it's a correct usage. Btw, smart pointers are no more than a syntactic sugar.
I guess this is not correct way to use the smart pointer.
ReplyDeleteThis does not differ from the variant with a regular global pointer, without any reference counting at all.
T * ptr;
// thread 1
ptr->SomeMethod(); //The problem is the same - who is freeing it at this time?
To use it properly, we have to increment the reference counter safely in the thread, who is already an owner of the resource :
// thread 1
shared_ptr ptr(new T);
// now this thread is a sole owner of the resource
// lets share it with another thread.
shared_ptr ptr2 = ptr;// The counter was incremented in this thread - nobody could free it.
CreateThreadWithContext(Thread2Func, ptr.get());
// thread 2
void Thread2Func(T * p)
{
shared_ptr ptr(p, false);// false means NOT to increment reference counter again
ptr->SomeMethod();// You can use it safely, You are already an owner, nobody can free it.
}// destructor of shared_ptr decrement reference counter, and free the resource, if this thread is its last owner.
It may be enough for safe life-time management.
But your idea is interesting as a access point to some resource.
> I guess this is not correct way to use the smart pointer.
ReplyDeleteIt's an incorrect way to use a reference counting algorithm, if it does not support such usage. However, if it does support such usage, then it's a correct usage.
Btw, smart pointers are no more than a syntactic sugar.
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