From: John Torjo (john.lists_at_[hidden])
Date: 2004-09-29 01:47:51
Ben Hutchings wrote:
> John Torjo <john.lists_at_[hidden]> wrote:
>
>>Ben Hutchings wrote:
>
> <snip>
>
>>>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.
>
>
> Thanks for that. Supposing that expired() wrongly returns false
> because use_count is really 0 but it gets an old non-zero value.
> Then lock() attempts to construct a shared_ptr:
>
> calls shared_ptr<T>::shared_ptr<T>(weak_ptr<T> const &)
> calls detail::shared_count::shared_count(weak_count const &)
> calls detail::sp_counted_base::add_ref_lock()
> calls detail::atomic_conditional_increment(
> long volatile &)
> either reads the count as 0
> or calls _InterlockedCompareExchange
> fails and returns 0
> then loops around and reads the count as 0
> returns 0
> throws bad_weak_ptr
> catches bad_weak_ptr
> returns 0
>
Yes indeed.
It was my mistake. I failed to see the flow when shared_ptr gets
constructed from weak_ptr.
So, I was wrong from the beginning. Sorry.
But now, by looking again at atomic_conditional_increment. I'm not a
threading expert, but it seems pretty costly as it is implemented now.
Could it not go multiple times (n>=2) thorughout the for?
Current implementation:
inline long atomic_conditional_increment(long volatile & value)
{
for(;;)
{
long tmp = value;
if(tmp == 0) return 0;
if(InterlockedCompareExchange(&value, tmp + 1, tmp) == tmp)
return tmp + 1;
}
}
Couldn't it become:
inline long atomic_conditional_increment(long volatile & value)
{
long tmp = value;
if(tmp == 0) return 0;
long new_val = InterlockedCompareExchange(&value, tmp + 1, tmp);
if(new_val == tmp) return tmp + 1;
else return InterlockedIncrement(&value);
}
Best,
John
--
John Torjo
-- john_at_[hidden]
Contributing editor, C/C++ Users Journal
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