Python GIL and globals

Question

In python, I have a global variable defined that gets read/incremented by different threads. Because of the GIL, will this ever cause problems without using any kind of locking mechanism?

Answer 1

The GIL only requires that the interpreter completely executes a single bytecode instruction before another thread can take over. However, there is no reason to assume that an increment operation is a single instruction. For example:

>>> import dis
>>> dis.dis(compile("x=753","","exec"))
  1           0 LOAD_CONST               0 (753)
              3 STORE_NAME               0 (x)
              6 LOAD_CONST               1 (None)
              9 RETURN_VALUE
>>> dis.dis(compile("x+=1","","exec"))
  1           0 LOAD_NAME                0 (x)
              3 LOAD_CONST               0 (1)
              6 INPLACE_ADD
              7 STORE_NAME               0 (x)
             10 LOAD_CONST               1 (None)
             13 RETURN_VALUE

As you can see, even these simple operations are more than a single bytecode instruction. Therefore, whenever sharing data between threads, you must use a separate locking mechanism (eg, threading.lock) in order to maintain data consistency.

Answer 2

Yes, multithreading without locking almost always causes problems, with or without a GIL.