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.