Dan Sugalski wrote:
For a copying collector to work, all the mutators must be blocked, and arguably all readers should be blocked as well.
True of non-moving collectors, too. [...]
Some of what I've written up addresses why. [...] I'll send that section when I get out of the office.
Consider this simple object graph:
Root set = { &A, &B }
[ A=NULL]
[ B=&C ] ---> [ C=....]Collection begins in thread 1 and marks A as reachable before being preempted by thread 2:
[xA=NULL]
[ B=&C ] ---> [ C=....]Thread 2 sets A to &C, and nullifies B before being preempted again by thread 1:
[xA=&C ] ---> [ C=....]
[ B=NULL]Thread 1 marks B as reachable:
[xA=&C ] ---> [ C=....]
[xB=NULL]The mark phase complete, thread 1 sweeps:
[ A=&???] ---> ???
[ B=NULL]C was not marked reachable (although it was) and was thus erroneously collected, leaving a dangling pointer. This problem applies equally to copying and mark-sweep collectors.
—
Gordon Henriksen [EMAIL PROTECTED]
