Race Conditions, and Mutexes¶

Mother of All Race conditions: Integer Increment ¶

code/race-condition.py¶

import threading

integer = 0
ntimes = 10000000

def increment_background():
    global integer, ntimes
    for _ in range(ntimes):
        integer += 1

t = threading.Thread(target=increment_background,
                     daemon=True)    # <----- mark as daemon
t.start()

for _ in range(ntimes):
    integer += 1

print('final value:', integer)

$ python code/race-condition.py
final value: 14784035

⟶ Load Modify Store Conflict

Load Modify Store Conflict ¶

CPU A		CPU B		Memory
Instr	Loc	Instr	Loc	Glob
load	42			42
	42	load	42	42
inc	43			42
	43	inc	43	42
	43	store	43	43
store	43		43	43

Solution: Mutex (Explicit Acquire/Release)¶

Mutex (for “MUTual EXclusion”), short: Lock
Best analogy: toilet door lock

code/mutex.py¶

import threading

integer = 0
ntimes = 10000000
lock = threading.Lock()

def increment_background():
    global integer, ntimes
    for _ in range(ntimes):
        lock.acquire()
        integer += 1
        lock.release()

t = threading.Thread(target=increment_background,
                     daemon=True)    # <----- mark as daemon
t.start()

for _ in range(ntimes):
    lock.acquire()
    integer += 1
    lock.release()

print('final value:', integer)

$ python code/mutex.py
final value: 20000000

Solution: Mutex (`with` Statement)¶

Calling lock.acquire() and lock.release() manually
⟶ what if exception is raised in the middle? (Well, integer increments don’t usually raise one; you get the point though.)

⟶ This is what the with statement is there fore (threading.Lock can act as a context manager)

code/mutex-with.py¶

import threading

integer = 0
ntimes = 10000000
lock = threading.Lock()

def increment_background():
    global integer, ntimes
    for _ in range(ntimes):
        with lock:
            integer += 1


t = threading.Thread(target=increment_background,
                     daemon=True)    # <----- mark as daemon
t.start()

for _ in range(ntimes):
    with lock:
        integer += 1

print('final value:', integer)

Dependencies ¶