run two thread concurrently and both of them manipulate single variable

Question

it's my code:

import threading
x=0
class a(threading.thread)
    def run(self):
        global x
        for i in range(1000000):
             x+=1

class b(threading.thread)
    def run(self):
        global x
        for i in range(1000000):
             x-=1
def run():
    a().start()
    b().start()
    //after both thread done
    print x
run()

i expect this show me 0 (x=0) but every time i run it the result is pretty different(less than zero)
what's wrong with it?

Answer 1

Race conditions. The actual operation of x += 1 is roughly:

1. Load the value of x
2. Compute x + 1
3. Store the computed value to x

Except with threading, you might get preempted by the other thread after step 1 and before step 3 (whether it's before or after 2 doesn't matter). If the other thread sees the unincremented value, decrements that, then you store your incremented value before it stores the decremented value, you just dropped an increment; if they store before you do, you dropped a decrement.

You need to lock access to shared variables to ensure that the operation behaves atomically:

import threading
x=0
xlock = threading.Lock()
class a(threading.Thread):
    def run(self):
        global x
        for i in range(1000000):
             with xlock:
                 x+=1

class b(threading.Thread):
    def run(self):
        global x
        for i in range(1000000):
             with xlock:
                 x-=1

This may introduce quite a bit of overhead, so other alternatives that touch the shared variable less may be better options (at the expense of having different behaviors).