draft proposal for ref counting in D

[Walter Bright]

On 6/29/2013 12:36 AM, 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.

Ah, don't rip up your book! (I cut the back off of mine and scanned it, but I no 
longer care to store the thousands of pounds of books I have anymore, and I like 
that my whole library fits on my laptop now!).

 >
 >>
 >> 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.
 >
 >

Ok, I see. Let me think about it some more.