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

From: John Torjo (john.lists_at_[hidden])
Date: 2004-09-25 06:40:32

Alexander Terekhov wrote:

> John Torjo wrote:
> [...]
> 
>>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.
> 
> 
> No barriers needed for its lock() call (and shared_ptr-from-weak_ptr
> ctor). It can't see 'false' zero unless you violate the "basic" thread-
> safety contract. Value consistency is ensured by conditional increment 

which is?

> using 'naked' (w/o any barriers) CAS (LL/LR-SC aside for a moment) IFF 
> the observed 'old' value is not zero. 
> 

(after about 3 hours of reading different threads on google)

I still think I'm right ;)

Here's my scenario:
(remember - we're talking about:
http://www.pdimov.com/cpp/shared_count_x86_exp2.hpp)

class test { ... };

[thread1]
shared_ptr<test> p1( new test); // refcount = 1

---------------- thread switch
[thread2]
shared_ptr<test> p2(p1); // refcount = 2
weak_ptr<test> weak(p2);
p2 = shared_ptr<test>(); // refcount = 1

------------------thread switch
[thread1]
p1 = shared_ptr<test>(); // refcount = 0 - object gets destroyed
---------------- thread switch
[thread2]

// here - you need InterlockedCompareExchange
// to realize that refcount is 0 (zero)
shared_ptr<test> p3 = weak.lock();

Also, see my other post to Peter Dimov.

Best,
John

-- 
John Torjo
-- john_at_[hidden]
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