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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-09-24 12:46:11

John Torjo wrote:
> 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.

atomic_read is only called by use_count. You shouldn't be using use_count to 
check whether a weak_ptr is still valid, because this can never be thread 
safe, regardless of the implementation of atomic_read. Use weak_ptr::lock.