Rainer Schuetze wrote:
On 29.06.2013 06:38, Walter Bright wrote:
>
> On 6/27/2013 11:33 AM, Rainer Schuetze wrote:
>> On 27.06.2013 19:04, Walter Bright wrote:
>>>> IIRC you also have the GC handbook book on your shelf. Check the
>>>> chapters on RC, especially algorithm 18.2 "Eager reference counting
>>>> with
>>>> CompareAndSwap is broken".
>>>
>>> I have the book, but it is the first edition and there's no chapter 18
>>> in it :-(
>>
>> I can remove the dust from my scanner to copy the 3 mostly relevant
>> pages and send them to you.
>>
>>
I tried to scan it yesterday, but got large black bar at the fold (don't know if
this the correct term) that eraased the first inch of text. I would have to rip
the book apart to get better results.
>
> I understand the issue (I think), but I can't think of a case where the
> ref count would be 1 when this happens.
>
>
Consider a global shared reference R that holds the last reference to an object
O. One thread exchanges the reference with another reference P while another
thread reads the reference into S.
shared(C) R = O; ; refcnt of O is 1
in pseudo-assembly missing null-checks:
Thread1 (R = P) Thread2 (S = R)
mov ecx,[R]
; thread suspended
mov eax,[P]
inc [eax].refcnt
mov ebx,[R]
mov [R],eax
dec [ebx].refcnt ; refcnt of O now 0
jnz done
call delete_ebx
; thread resumed
inc [ecx].refcnt
done:
The increment on [ecx].refcnt modifies garbage.
