Fix missing write barriers of imemo_fields in Ractor copy/move traversal by jhawthorn · Pull Request #13783 · ruby/ruby · GitHub | Latest TMZ Celebrity News & Gossip | Watch TMZ Live
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Fix missing write barriers of imemo_fields in Ractor copy/move traversal #13783

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Fix missing write barriers of imemo_fields in Ractor copy/move traversal #13783

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Found by wbcheck. This fixes two cases where write barriers could be missed when Ractor copying/moving objects with generic fields.

  • When we move a geniv object, the new object needs a write barrier because it gains a reference to the fields. (incremental GC could happen as during inserting the new key into the table)
  • When we're copying a geniv object, the write barrier needs to come from the fields object rather than the object itself.

cc @byroot

jhawthorn added 2 commits July 2, 2025 23:45
@jhawthorn
Fix ractor imemo fields write barrier parent
3675ba6 
    $ RUBY_GC_LIBRARY=wbcheck ./miniruby -e 's = String.new; s.instance_variable_set(:@x, []); Ractor.make_shareable(s, copy: true)'

    WBCHECK ERROR: Missed write barrier detected!
      Parent object: 0x7ba8162dc890 (wb_protected: true)
        rb_obj_info_dump: 0x00007ba8162dc890 T_IMEMO/<fields>
      Reference counts - snapshot: 2, writebarrier: 0, current: 2, missed: 1
      Missing reference to: 0x7ba8162dcad0
        rb_obj_info_dump: 0x00007ba8162dcad0 T_ARRAY/Array [E ] len: 0 (embed)

    WBCHECK SUMMARY: Found 1 objects with missed write barriers (1 total violations)
@jhawthorn
Fix missed write barrier on Ractor send move
8eee9e1 
When moving a "generic IV" object, we need a write barrier to the fields
object.

   WBCHECK ERROR: Missed write barrier detected!
     Parent object: 0x7c913641d1a0 (wb_protected: true)
       rb_obj_info_dump: 0x00007c913641d1a0 T_ARRAY/Array [E ] len: 10 (embed)
     Reference counts - snapshot: 1, writebarrier: 0, current: 2, missed: 1
     Missing reference to: 0x7bf1364e56d0
       rb_obj_info_dump: 0x00007bf1364e56d0 T_IMEMO/<fields>
jhawthorn
jhawthorn commented Jul 3, 2025
View reviewed changes
ractor.c
@@ -1681,7 +1681,7 @@ obj_traverse_replace_i(VALUE obj, struct obj_traverse_replace_data *data)
uint32_t fields_count = RSHAPE_LEN(RBASIC_SHAPE_ID(obj));
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I feel like up here I need to also make a change so that the complex fields also have the correct write barrier, but I want to make a test first that surfaces that to wbcheck

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@byroot byroot byroot approved these changes

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