While I'm familiar with concurrent programming concepts such as mutexes and semaphores, I have never understood how they are implemented at the assembly language level.

I imagine there being a set of memory "flags" saying:

• lock A is held by thread 1
• lock B is held by thread 3
• lock C is not held by any thread
• etc

But how is access to these flags synchronized between threads? Something like this naive example would only create a race condition:

  mov edx, [myThreadId]
wait:
  cmp [lock], 0
  jne wait
  mov [lock], edx
  ; I wanted an exclusive lock but the above 
  ; three instructions are not an atomic operation :(