Re: [boost] Re: Weak ref. via atomicity (RE: Smart pointers:...)

From: Pavel Vasiliev (pavel_at_[hidden])
Date: 2003-02-13 09:28:35

Alexander Terekhov wrote:

[...]
> Pavel Vasiliev wrote:
>> I place here the corrected version of my previous code.

> Well,

> Given: two threads -- A and B, 
>        thread A has "strong" one,
>        thread B has "weak" one,
>        strong_count == 1, weak_count == 2.

> Thread A, in release_strong:
>        atomic_decrement(&strong_count) == 0,
> Thread B, in acquire_strong_from_weak:
>        lock,
>        see atomic_increment(&strong_count) > 0,
>        (strong_count == 1, weak_count == 2)
>        return true (and unlock),
>        ...
>        enter release_strong()
>        atomic_decrement(&strong_count) == 0,
>        enter strong_refs_lost(),
>        see strong_count == 0,
>        set strong_count < 0,
>        unlock,
>        destroy,
>        enter release_weak(),
>        atomic_decrement(&weak_count) == 1,
>        (strong_count < 0, weak_count == 1)
>        ...
>        enter release_weak(),
>        atomic_decrement(&weak_count) == 0,
>        destruct_self()
> Thread A, enter strong_refs_lost():

Yeah... This attempt failed :-(. Thanks for preventing me from getting
into troubles!

However,

> WHA.. Nah, Lukashenko retires and establishes peace and democracy in
> the Middle East. ``Not bad.'' ;-)

I'd better leave my code as is. Or even dereference a NULL-pointer :-).

[...]
> pthread_refcount_decrement()         // with msync for proper mut.obj-cleanup
> pthread_refcount_decrement_nomsync() // without any msync whatsoever

"nomsync" here stands for "no memory view update even when counter drops
to 0"? If yes, then you probably should not use it in

>  void release_weak() {
>    int status = pthread_refcount_decrement_no_msync(&weak_count);
>    if (PTHREAD_REFCOUNT_DROPPED_TO_ZERO == status)
>      destruct_self();
>  }

Consider the following:
> Given: two threads -- A and B,
>        thread A has "strong" one,
>        thread B has "weak" one,
>        strong_count == 1, weak_count == 2.

Thread A releases the last "strong" reference.
destruct_object() deletes p_object AND modifies the refs instance
(e.g. sets p_object to NULL for debug reasons).

strong_count == 0, weak_count == 1

Thread B releases the last "weak" reference.
release_weak() calls destruct_self() on expired memory view.

"Or am I just missing and/or misunderstanding something?" :-)

Pavel