[webkit-dev] MessagePorts and garbage collection

Drew Wilson atwilson at google.com
Wed May 6 18:41:20 PDT 2009

Following up. I think I have my head around how Worker GC is happening (I
may start another thread about that, as it looks like there's some cases
where the thread won't be shut down, but the general design is sound).

MessagePort GC is a little trickier, because we need to detect when both
sides have no external references, based on this part of the HTML5 spec:

> [...] a message port can be received, given an event listener, and then
> forgotten, and so long as that event listener could receive a message, the
> channel will be maintained.
> Of course, if this was to occur on both sides of the channel, then both
> ports would be garbage collected, since they would not be reachable from
> live code, despite having a strong reference to each other.
>From looking at the code in bindings/js, it looks like I've got two tools to
manage object reachability:

1) I can tell when my object is reachable (during a GC) because mark() will
be invoked on it.
2) I can force my object to stay active (as long as the owning context is
active) by making it an ActiveDOMObject and returning true from
hasPendingActivity() (which seems like it does nothing but invoke mark() on
the object).

So, #2 lets me keep an object alive, but to implement the spec, I need to be
able to detect when my object has no more references, without actually
having it get garbage collected. If I can do that, then I can build my own
distributed state mechanism to allow me to determine when it's safe to GC
the objects.

I'm looking through the JSC::Collector code, and I didn't see anything that
did exactly what I want, but there are probably some things that we could do
with protect() to enable this. Has anyone else had to do anything like what
I describe above? It's not exactly even a multi-thread issue, as it seems
like this problem would occur even with just a single thread.


2009/5/6 Drew Wilson <atwilson at google.com>

> Thanks, this puts me on the right track. I've had a bunch of discussions
> with the Chrome folks on how we'd track MessagePort reachability in Chrome,
> but I'd hoped that the problem might be simpler in WebKit since we had
> direct access to the data structures cross-thread. The existence of separate
> GC heaps means it's not particularly simpler after all.
> -atw
> 2009/5/6 Maciej Stachowiak <mjs at apple.com>
>> On May 6, 2009, at 1:53 PM, Drew Wilson wrote:
>> OK, that's good to know (it only supports document contexts) - clearly
>> some work has been done to prepare for multi-thread usage (for example, the
>> core data structure is a thread-safe MessageQueue).
>> I'm quite happy to drive this design (in fact, I'm in the middle of this
>> now) but I would like to make sure I understand in general what the correct
>> approach is for managing GC-able objects that are accessed cross-thread - I
>> haven't been able to find any documentation (outside of the code itself).
>> Is the right approach to use JSLock when manipulating cross-thread
>> linkage? I'll write up a quick document to describe the approach I'm taking,
>> but I'd like to understand your concerns about deadlocks. So long as we have
>> only a single shared per-channel mutex, and we never grab any other locks
>> (like JSLock) after grabbing that mutex, we should be OK. Are there other
>> locks that may be grabbed behind the scenes that I should be aware of?
>> JSLock is not the right approach. Workers have their own completely
>> separate GC heap. JSLock only locks the current context group's heap. It
>> will not prevent collection in other heaps.
>> I don't know exactly what the right approach is. Ultimately it's a
>> distributed GC problem, both for our split-heap multithreading and for an
>> approach that used processes for workers. And distributed GC is hard.
>> However, Worker itself has a similar issue, since it can be kept alive
>> either from the inside or the outside reference. You could look at how that
>> problem was solved.
>>  - Maciej
>> -atw
>> 2009/5/6 Alexey Proskuryakov <ap at webkit.org>
>>> 06.05.2009, в 21:38, Drew Wilson написал(а):
>>>  It looks like the JSC collection code relies on JSLock to lock the heap
>>>> - I'm guessing that I'll need to explicitly grab the JSLock whenever I'm
>>>> manipulating the linkage between the two ports, is that correct? Or is there
>>>> a different/better way to handle situations like this?
>>> The JavaScriptCore implementation of MessagePorts only supports document
>>> contexts (i.e., it only works on main thread).
>>> As mentioned earlier, the first thing needed to implement MessagePorts in
>>> workers is a design of how they can be passed around without breaking GC. It
>>> is likely that taking a lock whenever atomicity is desired will cause
>>> deadlocks.
>>> - WBR, Alexey Proskuryakov
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