Do root scanning in scan_vm_specific_roots

We rely on scan_vm_specific_roots to reach all stacks via the following
path:

    VM -> ractors -> threads -> fibers -> stacks

Author: wks

gc/mmtk/mmtk.c

@@ -203,7 +203,13 @@ rb_mmtk_get_mutators(void (*visit_mutator)(MMTk_Mutator *mutator, void *data), v
 static void
 rb_mmtk_scan_gc_roots(void)
 {
-    // rb_gc_mark_roots(rb_gc_get_objspace(), NULL);
+    struct objspace *objspace = rb_gc_get_objspace();
+
+    // FIXME: Make `rb_gc_mark_roots` aware that the current thread may not have EC.
+    // See: https://github.com/ruby/mmtk/issues/22
+    rb_gc_worker_thread_set_vm_context(&objspace->vm_context);
+    rb_gc_mark_roots(objspace, NULL);
+    rb_gc_worker_thread_unset_vm_context(&objspace->vm_context);
 }
 
 static int
@@ -242,18 +248,6 @@ rb_mmtk_scan_objspace(void)
     }
 }
 
-static void
-rb_mmtk_scan_roots_in_mutator_thread(MMTk_VMMutatorThread mutator, MMTk_VMWorkerThread worker)
-{
-    if (mutator->gc_mutator_p) {
-        struct objspace *objspace = rb_gc_get_objspace();
-
-        rb_gc_worker_thread_set_vm_context(&objspace->vm_context);
-        rb_gc_mark_roots(objspace, NULL);
-        rb_gc_worker_thread_unset_vm_context(&objspace->vm_context);
-    }
-}
-
 static void
 rb_mmtk_scan_object_ruby_style(MMTk_ObjectReference object)
 {
@@ -402,7 +396,6 @@ MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_get_mutators,
     rb_mmtk_scan_gc_roots,
     rb_mmtk_scan_objspace,
-    rb_mmtk_scan_roots_in_mutator_thread,
     rb_mmtk_scan_object_ruby_style,
     rb_mmtk_call_gc_mark_children,
     rb_mmtk_call_obj_free,

gc/mmtk/mmtk.h

@@ -61,8 +61,6 @@ typedef struct MMTk_RubyUpcalls {
     void (*get_mutators)(void (*visit_mutator)(MMTk_Mutator*, void*), void *data);
     void (*scan_gc_roots)(void);
     void (*scan_objspace)(void);
-    void (*scan_roots_in_mutator_thread)(MMTk_VMMutatorThread mutator_tls,
-                                         MMTk_VMWorkerThread worker_tls);
     void (*scan_object_ruby_style)(MMTk_ObjectReference object);
     void (*call_gc_mark_children)(MMTk_ObjectReference object);
     void (*call_obj_free)(MMTk_ObjectReference object);

gc/mmtk/src/abi.rs

@@ -315,8 +315,6 @@ pub struct RubyUpcalls {
     ),
     pub scan_gc_roots: extern "C" fn(),
     pub scan_objspace: extern "C" fn(),
-    pub scan_roots_in_mutator_thread:
-        extern "C" fn(mutator_tls: VMMutatorThread, worker_tls: VMWorkerThread),
     pub scan_object_ruby_style: extern "C" fn(object: ObjectReference),
     pub call_gc_mark_children: extern "C" fn(object: ObjectReference),
     pub call_obj_free: extern "C" fn(object: ObjectReference),

gc/mmtk/src/scanning.rs

@@ -5,7 +5,7 @@ use crate::{upcalls, Ruby, RubySlot};
 use mmtk::scheduler::{GCWork, GCWorker, WorkBucketStage};
 use mmtk::util::{ObjectReference, VMWorkerThread};
 use mmtk::vm::{ObjectTracer, RootsWorkFactory, Scanning, SlotVisitor};
-use mmtk::{Mutator, MutatorContext};
+use mmtk::Mutator;
 
 pub struct VMScanning {}
 
@@ -67,14 +67,13 @@ impl Scanning<Ruby> for VMScanning {
     }
 
     fn scan_roots_in_mutator_thread(
-        tls: VMWorkerThread,
-        mutator: &'static mut Mutator<Ruby>,
-        mut factory: impl RootsWorkFactory<RubySlot>,
+        _tls: VMWorkerThread,
+        _mutator: &'static mut Mutator<Ruby>,
+        mut _factory: impl RootsWorkFactory<RubySlot>,
     ) {
-        let gc_tls = unsafe { GCThreadTLS::from_vwt_check(tls) };
-        Self::collect_object_roots_in("scan_thread_root", gc_tls, &mut factory, || {
-            (upcalls().scan_roots_in_mutator_thread)(mutator.get_tls(), tls);
-        });
+        // Do nothing.  All stacks (including Ruby stacks and machine stacks) are reachable from
+        // `rb_vm_t` -> ractor -> thread -> fiber -> stacks.  It is part of `ScanGCRoots` which
+        // calls `rb_gc_mark_roots` -> `rb_vm_mark`.
     }
 
     fn scan_vm_specific_roots(tls: VMWorkerThread, factory: impl RootsWorkFactory<RubySlot>) {