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

From: John Torjo (john.lists_at_[hidden])
Date: 2004-09-24 11:54:11

Peter Dimov wrote:

> John Torjo wrote:
> 
>>Hi Peter,
>>
>>I think there is a bug in your Interlocked* implementation of
>> shared_ptr (http://www.pdimov.com/cpp/shared_count_x86_exp2.hpp).
>>
>>In atomic_read you have:
>>inline long atomic_read(long volatile const  & value)
>>{
>>    return value;
>>}
>>
>>I don't believe this is thread-safe.
> 
> 
> (repost)
> 
> On x86/IA32, I believe that it's as thread safe as you can get. Note that
> all count updates go through Interlocked* calls. On IA32, plain reads can
> only return a somewhat "surprising" value when interleaved with plain
> writes, and even then, the element of surprise is limited. ;-) 
> 
> 
I kind of wonder.

Here's my test scenario, which I think is quite effective ;)

The main thread creates a window. Then it internally has a shared 
pointer to it.
Other threads might want access to this window. Each other thread keeps 
a weak_pointer to it.

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.

Best,
John

-- 
John Torjo
-- john_at_[hidden]
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