From: Martin Hart <martin@...>
Date: 2004-05-28T01:47:31+09:00
Subject: Mutex re-entrancy

Hi all,

I am using a Mutex to protect a function, but I need to call into the
function again from the same thread without deadlocking:

class A
  def initialize
    @m = Mutex.new
  end
  def a
    @m.synchronize { b }
  end
  def b
    @m.synchronize { puts "hello" }
  end
end
A.new.a #-> deadlock

It seems to me that to achieve this, all i need to do is track which thread
has the mutex locked, and allow access if the same thread requests it again:

class Mutex2
  def initialize
    @m = Mutex.new
    @l = nil
  end
  def synchronize
    if @l == Thread.current
      yield
    else
      @m.synchronize {
        @l = Thread.current
        begin
          yield
        ensure
          @l = nil
        end
      }
    end
  end
end

q1) am i reinventing the wheel and/or making it square?
q2) is this safe?

TIA
Martin