[Python-Dev] GIL removal question

[Stefan Behnel]

Guido van Rossum, 12.08.2011 23:38:
> On Fri, Aug 12, 2011 at 12:57 PM, Rene Nejsum wrote:
>> I think I understand the background and need for GIL. Without it Python
>> programs would have been cluttered with lock/synchronized statements and
>> C-extensions would be harder to write.
>
> No, sorry, the first half of this is incorrect: with or without the
> GIL *Python* code would need the same amount of fine-grained locking.
> (The part about C extensions is correct.) I am butting in because this
> is a common misunderstanding that really needs to be squashed whenever
> it is aired -- the GIL does *not* help Python code to synchronize. A
> thread-switch can occur between any two bytecode opcodes. Without the
> GIL, atomic operations (e.g. dict lookups that doesn't require
> evaluation of __eq__ or __hash__ implemented in Python) are still
> supposed to be atomic.

And in this context, it's worth mentioning that even C code can be bitten 
by the GIL being temporarily released when calling back into the 
interpreter. Only plain C code sequences safely keep the GIL, including 
many (but not all) calls to the C-API.

Stefan