From: John Torjo (john.lists_at_[hidden])
Date: 2004-09-25 05:23:40
Peter Dimov wrote:
> John Torjo wrote:
>>Peter Dimov wrote:
>>>John Torjo wrote:
>>>>I think there is a bug in your Interlocked* implementation of
>>>>In atomic_read you have:
>>>>inline long atomic_read(long volatile const & value)
>>>> return value;
>>>>I don't believe this is thread-safe.
>>>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
>>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.
Lets take a look at weak_ptr::lock():
wnd_shared_ptr<T> lock() const // never throws
// optimization: avoid throw overhead
catch(bad_wnd_weak_ptr const &)
// Q: how can we get here?
// A: another thread may have invalidated r after the
use_count test above.
// optimization: avoid try/catch overhead when single threaded
return expired()? wnd_shared_ptr<element_type>():
And lets look at expired()
bool expired() const // never throws
return pn.use_count() == 0;
Now, see what I mean?
-- John Torjo -- john_at_[hidden] Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/ -- v1.4 - save_dlg - true binding of your data to UI controls! + easily add validation rules (win32gui/examples/smart_dlg)
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