[Cython] GIL handling C code

Stefan Behnel stefan_ml at behnel.de
Tue Feb 28 22:08:54 CET 2012

mark florisson, 28.02.2012 21:20:
> On 28 February 2012 19:58, Stefan Behnel wrote:
>> mark florisson, 28.02.2012 16:35:
>>> Basically, the cleanup code only needs a matching release because the
>>> corresponding acquire is in EnsureGILNode, which wraps the function
>>> body in case of a nogil function with a 'with gil' block. Any changes
>>> to the conditions in FuncDefNode will have to be reflected by the code
>>> that does that wrapping. Changing the refnanny macro for the cleanup
>>> code will not avail anything, as the GIL is already ensured.
>> Regarding the "with gil" code, ISTM that the "finally" code in the with_gil
>> test is being duplicated. I noticed this when I moved the refnanny's GIL
>> state into a block local variable and that broke the C code. Basically, the
>> with-gil block had declared the variable in its own block, but was then
>> trying to access that variable in a second finally clause, further down and
>> outside of the with-gil block.
>> Looks like a bug to me.
> That's not a bug, that's how it is implemented. At setup, a variable
> __pyx_gilstate_save is declared, which is also used for teardown. Any
> with GIL blocks use C block scoping to do the same thing. The with
> block is itself a try/finally, so you get a try/finally wrapped in a
> try/finally. The code uses try/finally for it's way of trapping
> control flow, allowing some code to execute and resuming control flow
> afterwards.

Ok, so what really got me confused is that the code used the variable
"acquire_gil_for_refnanny_only" in places that didn't have anything to do
with the refnanny. Similarly, "acquire_gil_for_var_decls_only" was used for
cleanup even though the GIL had already been released if this flag was set,
way before the end of the function.

I think I fixed both issues in the patch I attached. At least, it still
passes the gil related tests and doesn't raise any C compiler warnings
about the GIL state variable being unused.

Does this look about right?

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