[Python-Dev] PyWeakref_GetObject() borrows its reference from... whom?

[Jim J. Jewett]

On Mon, Oct 10, 2016, at 14:04, MRAB wrote:
> Instead of locking the object, could we keep the GIL, but have it
> normally released?

> A thread could then still call a function such as PyWeakref_GetObject()
> that returns a borrowed reference, but only if it's holding the GIL. It
> would be able to INCREF the reference before releasing the GIL again.

So you need to get/release the GIL just to run a slightly faster function
that doesn't bother with an extra incref/defcref pair?  I think anyone
willing to make those changes would be willing to switch to a
non-borrowing version of that same function, and do an explicit
DECREF if that is really what they wanted.

On Tue, Oct 11, 2016 at 5:24 AM, Random832 <random832 at fastmail.com> wrote:
> So, what stops the other thread which never asks for the GIL from
> blowing away the reference? Or is this a special kind of lock that you
> can "assert isn't locked" without locking it for yourself, and
> INCREF/DECREF does so?

On Mon Oct 10 15:36:59 EDT 2016, Chris Angelico wrote:
> "assert isn't locked" is pretty cheap

Yeah, but so is INCREF/DECREF on memory that is almost certainly
in cache anyhow, because you're using the object right next to it.

The write part hurts, particularly when trying to use multiple cores
with shared memory, but any sort of indirection (even separating the
refcount from the object, to allow per-core counters) ... well, it
doesn't take much at all to be worse than INCREF/DECREF in even
the normal case, let alone amortized across the the "drat, this
object now has to be handled specially" cases.

Imagine two memory pools, one for "immortal" objects (such as
None) that won't be collected, and so don't need their memory
dirtied when you INCREF/DECREF.  Alas, now *every* INCREF and DECREF
has to branch on the address to tell whether or not it should be a
no-op.

-jJ

--

If there are still threading problems with my replies, please
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