[ruby-core:112038] [Ruby master Bug#19376] Ability to read ivars of non-frozen classes/modules across ractors is not thread-safe

25 Jan 2023

      Issue #19376 has been reported by luke-gru (Luke Gruber).

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Bug #19376: Ability to read ivars of non-frozen classes/modules across ractors is not thread-safe
https://bugs.ruby-lang.org/issues/19376

* Author: luke-gru (Luke Gruber)
* Status: Open
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
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This segfaults:

```ruby
class C
  class << self
    attr_accessor :val
  end
end

GO = false
rs = 100.times.map do
  Ractor.new do
    Thread.pass until GO
    1000.times do
      val = C.val
      p val
    end
  end
end
ts = 100.times.map do |i|
  Thread.new do
    Thread.pass until GO
    1000.times do
      C.val = Object.new.freeze
    end
  end
end

GO = true
ts.each(&:join)
rs.each(&:take)
```

It looks like a silly example, but it can crash even if the main thread doesn't spawn any more threads. Getting/setting ivars on objects in parallel without VM lock isn't safe, so for non-frozen shareable objects (basically classes and modules), it's unsafe to access their instance variables even if the values are shareable.

-- 
https://bugs.ruby-lang.org/