On 12.10.2013 04:16, inout wrote:
On Thursday, 10 October 2013 at 08:55:00 UTC, Robert Schadek
wrote:
On 10/10/2013 03:45 AM, Walter Bright wrote:
Rainer Schuetze wrote:
You have to put the lock around the pair of AddRef and Release, but if
the compiler already splits this into two function calls, this cannot
be done in the implementation.
I would imagine the counter to be manipulated with atomic_add_and_fetch
operations, so no locks are required.
On shared objects, yes. Local objects need no atomics at all.
Atomic increments/decrements are not good enough for shared references.
See this example from later in the discussion:
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.
