Code Inspection: Inconsistent order of taken locks

If your class has multiple resources protected with locks to achieve thread safety, JetBrains Rider analyzes possible execution paths in multi-threaded environment (assuming public API of the class can be used simultaneously by multiple threads) and the order of locks taken on such execution paths to find cycles leading to possible deadlocks at runtime, as illustrated in the example below. The warning message provides a detailed explanation and an example of a cycle that could be formed.

  
  class MultiThreadedComponent
  {
    private List<string> _resource1 = new();
    private List<string> _resource2 = new();

    public void PublicApi01()
    {
      lock (_resource1)
      lock (_resource2)
      {
        // do work
      }
    }

    public void PublicApi02()
    {
      lock (_resource2)
      {
        // do work
        HelperMethod02();
        // do work
      }
    }

    private void HelperMethod02()
    {
      // The expression is used in several lock statements with inconsistent execution order,
      // forming a cycle. This might lead to a possible deadlock if methods (properties)
      // of this type are executed concurrently by multiple threads.
      // Lock objects involved in the cycle: '_resource1', '_resource2'
      // Example of the deadlock situation:
      //  - Thread #1 executes 'PublicApi01', takes locks '_resource1' -> '_resource2'
      //  - Thread #2 executes 'PublicApi02', takes locks '_resource2' -> '_resource1'
      lock (_resource1)
      {
        // do work
      }
    }
  }
  
    

Last modified: 04 January 2022