Quick Answer

Avoid recursive mutex locking.

Understanding the Issue

Occurs when a thread attempts to lock a mutex it already holds, creating a deadlock situation.

The Problem

This code demonstrates the issue:

Ruby Error 
mutex = Mutex.new
mutex.synchronize do
  mutex.synchronize do
    # Deadlock!
  end
end

The Solution

Here's the corrected code:

Ruby Fixed 
# Solution 1: Use non-recursive locks carefully
mutex = Mutex.new
mutex.synchronize do
  # Critical section
end

# Solution 2: Use Monitor for reentrant locks
require "monitor"
monitor = Monitor.new
monitor.synchronize do
  monitor.synchronize do
    # This works (Monitor is reentrant)
  end
end

# Solution 3: Separate mutexes
read_lock = Mutex.new
write_lock = Mutex.new

Key Takeaways

Use Monitor for reentrant locking needs or restructure critical sections.