Make rb_gc_impl_writebarrier_remember Ractor-safe

rb_gc_impl_writebarrier_remember is not Ractor safe because it writes to
bitmaps and also pushes onto the mark stack during incremental marking.
We should acquire the VM lock to prevent race conditions.

In the case that the object is not old, there is no performance impact.
However, we can see a performance impact in this microbenchmark where the
object is old:

    4.times.map do
      Ractor.new do
        ary = []

        3.times { GC.start }

        10_000_000.times do |i|
          ary.push(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
          ary.clear
        end
      end
    end.map(&:value)

Before:

    Time (mean ± σ):     682.4 ms ±   5.1 ms    [User: 2564.8 ms, System: 16.0 ms]

After:

    Time (mean ± σ):      5.522 s ±  0.096 s    [User: 8.237 s, System: 7.931 s]

Co-Authored-By: Luke Gruber <luke.gruber@shopify.com>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

1 files changed, 12 insertions, 8 deletions

diff --git a/gc/default/default.c b/gc/default/default.c
index 9038a01e4e..d4e34b9d03 100644
--- a/gc/default/default.c
+++ b/gc/default/default.c
@@ -6110,15 +6110,19 @@ rb_gc_impl_writebarrier_remember(void *objspace_ptr, VALUE obj)
 
     gc_report(1, objspace, "rb_gc_writebarrier_remember: %s\n", rb_obj_info(obj));
 
-    if (is_incremental_marking(objspace)) {
-        if (RVALUE_BLACK_P(objspace, obj)) {
-            gc_grey(objspace, obj);
-        }
-    }
-    else {
-        if (RVALUE_OLD_P(objspace, obj)) {
-            rgengc_remember(objspace, obj);
+    if (is_incremental_marking(objspace) || RVALUE_OLD_P(objspace, obj)) {
+        int lev = RB_GC_VM_LOCK_NO_BARRIER();
+        {
+            if (is_incremental_marking(objspace)) {
+                if (RVALUE_BLACK_P(objspace, obj)) {
+                    gc_grey(objspace, obj);
+                }
+            }
+            else if (RVALUE_OLD_P(objspace, obj)) {
+                rgengc_remember(objspace, obj);
+            }
         }
+        RB_GC_VM_UNLOCK_NO_BARRIER(lev);
     }
 }