[Cython] GIL handling C code

mark florisson markflorisson88 at gmail.com
Tue Feb 28 22:22:19 CET 2012

On 28 February 2012 21:08, Stefan Behnel <stefan_ml at behnel.de> wrote:
> 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.

Yeah those names weren't very apt, they are useful for the setup code
but don't make sense for the teardown code :)

> 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?
> Stefan
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