Summary "Concurrent::ReentrantReadWriteLock" can incorrectly grant a write lock after one thread acquires the read lock 32,768 times. The lock stores a thread's local read and write hold counts in one integer. The low 15 bits are used for the read hold count, and bit 15 is used as "WRITE_LOCK_HELD". After 32,768 reentrant read acquisitions, the local read count crosses into the write-lock bit. "try_write_lock" then treats the thread as already holding a write lock and returns "true" without setting the global "RUNNING_WRITER" bit. This breaks the core mutual-exclusion guarantee: the caller is told it has a write lock, but other threads can still hold or acquire read locks at the same time. Version Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab Details The implementation uses a shared counter to track global readers/writers and a per-thread local counter to support reentrancy: READER_BITS = 15 WRITER_BITS = 14 WAITING_WRITER = 1 << READER_BITS RUNNING_WRITER = 1 << (READER_BITS + WRITER_BITS) MAX_READERS = WAITING_WRITER - 1 MAX_WRITERS = RUNNING_WRITER - MAX_READERS - 1 WRITE_LOCK_HELD = 1 << READER_BITS READ_LOCK_MASK = WRITE_LOCK_HELD - 1 WRITE_LOCK_MASK = MAX_WRITERS When a thread already holds a lock, "acquire_read_lock" increments "@HeldCount": if (held = @HeldCount.value) > 0 if held & READ_LOCK_MASK == 0 @Counter.update { |c| c + 1 } end @HeldCount.value = held + 1 return true end After 32,768 read acquisitions, the per-thread held count becomes "32768", which is equal to "WRITE_LOCK_HELD". Then "try_write_lock" returns success through its "already have a write lock" branch: def try_write_lock if (held = @HeldCount.value) >= WRITE_LOCK_HELD @HeldCount.value = held + WRITE_LOCK_HELD return true else # normal global writer acquisition path end end This branch does not set the global "RUNNING_WRITER" bit. Other threads therefore do not observe an active writer and can continue holding or acquiring read locks while the caller believes it owns the write lock. PoC #!/usr/bin/env ruby frozen_string_literal: true require 'concurrent/atomic/reentrant_read_write_lock' require 'concurrent/version' require 'thread' def wait_for_queue(queue, timeout_seconds) deadline = Process.clock_gettime(Process::CLOCK_MONOTONIC) + timeout_seconds loop do return queue.pop(true) rescue ThreadError return nil if Process.clock_gettime(Process::CLOCK_MONOTONIC) >= deadline sleep 0.001 end end puts "ruby=#{RUBY_DESCRIPTION}" puts "concurrent_ruby_version=#{Concurrent::VERSION}" puts "poc=ReentrantReadWriteLock read-depth overflow grants write lock without exclusivity" lock = Concurrent::ReentrantReadWriteLock.new other_reader_ready = Queue.new other_reader_stop = Queue.new other_reader = Thread.new do lock.acquire_read_lock other_reader_ready << :held other_reader_stop.pop end wait_for_queue(other_reader_ready, 1) puts "other_thread_holds_read_lock=true" depth = Concurrent::ReentrantReadWriteLock::WRITE_LOCK_HELD depth.times { lock.acquire_read_lock } held_count = lock.instance_eval { @HeldCount.value } counter_before = lock.instance_eval { @Counter.value } puts "main_thread_read_acquisitions=#{depth}" puts "main_thread_held_count=#{held_count}" puts "counter_before_try_write=#{counter_before}" puts "running_writer_bit_before=#{(counter_before & Concurrent::ReentrantReadWriteLock::RUNNING_WRITER) != 0}" write_granted = lock.try_write_lock counter_after = lock.instance_eval { @Counter.value } puts "try_write_lock_returned=#{write_granted}" puts "counter_after_try_write=#{counter_after}" puts "running_writer_bit_after=#{(counter_after & Concurrent::ReentrantReadWriteLock::RUNNING_WRITER) != 0}" third_reader_ready = Queue.new third_reader = Thread.new do lock.acquire_read_lock third_reader_ready << :acquired end third_reader_acquired = wait_for_queue(third_reader_ready, 0.25) == :acquired puts "new_reader_acquired_while_write_claimed=#{third_reader_acquired}" if write_granted && third_reader_acquired && (counter_after & Concurrent::ReentrantReadWriteLock::RUNNING_WRITER).zero? puts 'result=REPRODUCED write lock granted without setting global writer state' else puts 'result=NOT_REPRODUCED' end third_reader.kill other_reader_stop << :stop other_reader.kill Log evidence ruby=ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.arm64e-darwin25] concurrent_ruby_version=1.3.6 poc=ReentrantReadWriteLock read-depth overflow grants write lock without exclusivity other_thread_holds_read_lock=true main_thread_read_acquisitions=32768 main_thread_held_count=32768 counter_before_try_write=2 running_writer_bit_before=false try_write_lock_returned=true counter_after_try_write=2 running_writer_bit_after=false new_reader_acquired_while_write_claimed=true result=REPRODUCED write lock granted without setting global writer state Impact This breaks the write-lock exclusivity guarantee. After the overflow, a thread can be told it has acquired the write lock while other threads can still hold or acquire read locks, allowing races and inconsistent reads of protected mutable state. Credit Pranjali Thakur - depthfirst ("depthfirst.com" (http://depthfirst.com))