Re: [boost] Re: [shared_ptr] Interlocked* - possible bug?

From: John Torjo (john.lists_at_[hidden])
Date: 2004-09-27 15:09:17

Ben Hutchings wrote:

> John Torjo <john.lists_at_[hidden]> wrote:
> 
>>Alexander Terekhov wrote:
>>
>>
>>>John Torjo wrote:
>>>[...]
>>>
>>>
>>>>Whenever a thread (other than main) wants to access this window, it
>>>>will query the weak_pointer. The weak_pointer needs to know the
>>>>LATEST reference count in order to know if the weak pointer is still
>>>>valid. Thus, atomic_read that simply returns the value is not
>>>>enough.
> 
> 
> However, the weak_ptr doesn't just use atomic_read.  If atomic_read
> returns 0 that means the use count has dropped to 0 [*] and can never
> increase again, so it must be the latest version.  Otherwise weak_ptr
> makes a second test that is properly protected.  So it can never use
> an old value.

yes, that is quite right.

> 
> This is still unaccessible.  why don't you mail the code to Alexander
> and me (or to the list, in case anyone else is interested)?
> 
done that. See my other post.

> 
>>class test { ... };
>>
>>[thread1]
>>shared_ptr<test> p1( new test); // refcount = 1
>>
>>---------------- thread switch
> 
> 
> Switches between threads running on a single processor can't cause
> problems here, because the use of "volatile" ensures that the
> compiler won't reoder access to the use count and the processor will
> only reorder memory access in a way that is invisible to the code
> it's running.  There might conceivably be problems that occur when
> multiple threads run concurrently on multiple processors, in which

as a side note, my PC is already a dual-processor ;)

> case reordering by one processor can affect another.  So I'll ignore
> the "thread switches".

I just used the "thread switch" to make a point ;)

> 
> 
>>[thread2]
>>shared_ptr<test> p2(p1); // refcount = 2
>>weak_ptr<test> weak(p2);
>>p2 = shared_ptr<test>(); // refcount = 1
>>
>>------------------thread switch
>>[thread1]
>>p1 = shared_ptr<test>(); // refcount = 0 - object gets destroyed
>>---------------- thread switch
>>[thread2]
>>
>>// here - you need InterlockedCompareExchange
>>// to realize that refcount is 0 (zero)
>>shared_ptr<test> p3 = weak.lock();
> 
> 
> expired() may read the count as still being 1 and so return false,
> but I believe the wnd_shared_ptr constructor will catch the fact
> that the pointer really has expired.  Unfortunately I can't yet
> see the code to confirm this!
> 

By looking at the code, I would think not. But I may be wrong. Anyway, 
I've posted the code in my other post.

Best,
John

-- 
John Torjo
-- john_at_[hidden]
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