Paul Rubin <http://[EMAIL PROTECTED]> writes:
> AlFire <[EMAIL PROTECTED]> writes:
>> But I still can not believe that +=1 is not a thread safe operation.
>
> In CPython I believe it is thread safe, because of the global
> interpreter lock. Thread switches can happen only between bytecode
> executions, not in the middle of a bytecode, even though executing a
> bytecode can take several machine instructions.
It's safe when writing extensions in C (because of the GIL), but not
when writing Python code. Python switches threads after a number of
bytecode instructions, so Python code behaves somewhat like C
multithreaded code on a single CPU -- although there is no true
parallelism, you still have context switches at arbitrary places to
worry about, and still need locks because of them. Since the +=
operator is not compiled into a single bytecode instruction, it needs
the lock. x+=1 gets compiled into the following bytecode:
3 0 LOAD_GLOBAL 0 (x)
3 LOAD_CONST 1 (1)
6 INPLACE_ADD
7 STORE_GLOBAL 0 (x)
If two threads execute that code simultaneously, a thread switch could
easily occur some time between LOAD_GLOBAL and STORE_GLOBAL, causing
the variable to be incremented by 1 instead of by 2.
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