[phobos] Fwd: Re: Ruling out arbitrary cost copy construction?

[Michel Fortin]

Le 2010-11-01 à 12:30, David Simcha a écrit :

> Can someone please fill me in?  Despite Michael Fortin's repeated attempts
> to explain it, I still don't understand how the status quo could generate a
> race condition under reasonable, real-world scenarios.  As I understand it
> (please correct whatever reasoning is wrong):
> 
> 1.  Incrementing/decrementing a size_t is atomic (i.e. has no intermediate
> state) on x86 and probably just about any other arch.  It is not, however,
> guaranteed sequentially consistent unless you use the lock instruction.
> 

> 2.  Stopping the world to do GC acts as an implicit fence.  If it didn't,
> then GC would be horribly broken.  This means that all refcount
> increments/decrements that happened in the owner thread should be visible to
> the collecting thread upon starting a collection.
> 
> 3.  You'd need some kind of fence (explicit or implicit) on terminating GC
> anyhow, to make data structures updated by the GC thread visible to all
> threads, thus making any increment/decrement of the reference count field
> done in the GC thread visible to all threads.


The current GC stops other threads only during mark and sweep, it then restarts the other threads and after that it call destructors. That could be changed, but it'd stop the world for a little longer.

Is "i++" really atomic when i is a size_t? I though it was a read-modify-write operation. The read might be atomic, the write might be atomic, but the whole isn't. And in addition to atomicity, it needs to be sequentially consistent unless we change the GC to keep threads frozen while calling the destructors.

One option to avoid atomic ops would be to make the GC call destructors in the thread that owns the object, but for that you have to figure out which thread owns what and postpone destruction until that thread allocates new memory from the GC again. And this approach won't work for immutable objects anyway: they're implicitly shared.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/