[Python-Dev] Extension modules, Threading, and the GIL

["Martin v. Löwis"]

David Abrahams wrote:
> Python extension module B calls shared library Q and uses Q's callback
> interface.  Because some of the callbacks need to use the Python API,
> and *might* be invoked by threads, they must all acquire the GIL.

Wrong. If the code in B that calls Q does not allow threads, the 
callbacks don't need to reacquire the GIL.

> Problem: using B while A is loaded breaks A: because B has installed
> callbacks in Q that acquire the GIL, A must also release the GIL
> before calling into Q.

Can you please explain what a callback is? Can the callbacks occur in 
the context of a different thread, i.e. different from the one that has 
installed the callback?

If it is a true callback (i.e. Q will call B back while being called 
from B), then this won't interfere at all with A.

Regards,
Martin