Fix GCWorker ownership. #523
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This commit fixes an inconsistency in the VMCollection interface where a GCWorker is exposed to the binding via `Collection::spawn_worker_thread` as a `&GCWorker`, but later received from the binding via `memory_manager::start_worker` as a `&mut GCWorker`. The root cause is because GCWorker is wrongly owned by the scheduler. We make each GC worker thread the owner of its `GCWorker` struct, and we pass the `GCWorker` struct across API boundary as owning `Box`, fixing the interface. We isolate the part of `GCWorker` shared with the GC scheduler into a `GCWorkerShared` struct, and ensure the fields are properly synchronized. Particularly, we now use `AtomicRefCell` for `WorkerLocalStat`. `AtomicRefCell` allows it to be borrowed mutably by the GC worker and the mutator (via `harness_begin/end` methods) at different time. However, it is a known issue that in concurrent GC, it is possible for GC to happen when `harness_begin/end` is called. In that case, it will panic. Fixes: mmtk#522
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This was referenced Jan 27, 2022
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This PR fixes #522
We let GC worker threads own
GCWorker, and isolate shared parts intoGCWorkerShared. We also usedAtomicRefCellforWorkerLocalStat.This PR does not fix the potential data race between the mutator (
harness_beginandharness_end) and the GC. If such a race occurs, it will panic.I found that GC worker may access the GC scheduler (the
worker_groupfield) before the GC scheduler is fully consructed. Rust doesn't like this (and therefore there is anArc::get_mut_uncheckedinscheduler.rs:106), and that should be fixed in another PR.