[Python-Dev] Reworking the GIL

[Antoine Pitrou]

Martin v. Löwis <martin <at> v.loewis.de> writes:
> 
[gil_drop_request]
> Even if it is read from memory, I still wonder what might happen on
> systems that require explicit memory barriers to synchronize across
> CPUs. What if CPU 1 keeps reading a 0 value out of its cache, even
> though CPU 1 has written an 1 value a long time ago?
> 
> IIUC, any (most?) pthread calls would cause synchronization in that
> case, which is why applications that also use locks for reading:
> 
> http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap04.html#tag_04_10
> 
> Of course, on x86, you won't see any issues, because it's cache-coherent
> anyway.

I think there are two things here:
- all machines Python runs on should AFAIK be cache-coherent: CPUs synchronize
their views of memory in a rather timely fashion.
- memory ordering: writes made by a CPU can be seen in a different order by
another CPU (i.e. CPU 1 writes A before B, but CPU 2 sees B written before A). I
don't see how this can apply to gil_drop_request, since it's a single variable
(and, moreover, only a single bit of it is significant).

(there's an explanation of memory ordering issues here:
http://www.linuxjournal.com/article/8211)

As a side note, I remember Jeffrey Yasskin trying to specify an ordering model
for Python code
(see http://code.google.com/p/unladen-swallow/wiki/MemoryModel).

Regards

Antoine.