[second try in moving this discussion to cython-devel]
Dag Sverre Seljebotn, 12.08.2011 08:50:
On 08/12/2011 06:44 AM, Robert Bradshaw wrote:
On Thu, Aug 11, 2011 at 5:53 AM, Dag Sverre Seljebotn
<d.s.seljeb...@astro.uio.no> wrote:
On 08/11/2011 02:13 PM, Stefan Behnel wrote:
Dag Sverre Seljebotn, 11.08.2011 13:58:
On 08/11/2011 01:43 PM, Ting Zhou wrote:
here is my pyx file. When I compile it, it says "Calling gil-requiring
function not allowed without gil" at calling dabs(x[i]). Why my dabs
function is a gil-requiring function?
====================================
from cython.parallel import *
import numpy as np
cimport numpy as np
cimport cython
cdef double dabs(double x):
This should be
cdef double dabs(double x) nogil:
Note that Cython cannot infer this automatically. Even a trivial
function may require an exclusive global lock for some reason, and it's
common to use the GIL for that. So the programmer must be explicit here.
Are you still against this mini-CEP?:
with cython.global_lock():
...
Where global_lock() is GIL-requiring noop.
Just reading this, it's not immediately obvious what this means. (I
thought at first this was different syntax for "with gil"...)
True. "cython.synchronized"? The synchronized keyword in Java does not
quite the same thing but almost.
This
a) Improves code readability vastly. Having a critical section take effect
because of the *lack* of a keyword is just very odd to anyone who's not
shoulder deep in CPython internals
I'm not following you here. The only way to run into this is if you
have explicitly release it. Presumably you can learn both keywords at
the same time. Perhaps extern function could be nogil by default.
I'll try to explain again. Consider code like this:
cdef call_c():
magic_c_function(3)
This code is perfectly fine even if magic_c_function is not reentrant --
but that's hardly well documented! So it's conceivable that another
programmer comes along (who doesn't know the C library well) and decides
that "somebody has just been too lazy to add the nogil specifier" and slaps
it on -- at which point you have a bug.
This is more to the point in creating readable code IMO:
cdef call_c():
with cython.synchronized():
magic_c_function(3)
I think I'm as confused as Robert here. Is that the GIL or some separate lock?
If the latter, where would that lock be stored? Module-wide? While there
are certainly use cases for that, I think it's much more common to either
a) use the GIL or b) use an explicit lock at some well defined point, e.g.
at an object or class level.
I don't think we disagree when I say that locking should be explicit, but
that includes the "thing" that keeps the lock during its lifetime.
Also, consider your classical race conditions:
cdef init():
global resource
if resource == NULL:
resource = malloc(sizeof(resource_t)) ...
This is safe code.
Well, it's safe code as long as there is no Python code interaction in
between. Calling back into the interpreter may trigger a thread switch.
Admittedly, this is not obvious and tends to introduce bugs. I learned that
the hard way in lxml, actually twice.
If your "synchronised" directive refers to the GIL, then it would suffer
from that problem as well. I think that's very undesirable for an explicit
critical section statement.
Stefan
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