On Thu, Jun 09, 2022 at 11:24:48AM -0500, Eric Blake wrote:
> On Thu, Jun 09, 2022 at 03:27:36PM +0100, Richard W.M. Jones wrote:
>
> > > Post-patch:
> > > nbd> b = nbd.Buffer(10)
> > > nbd> c = h.aio_pread(b, 0)
> > > nbd> b._o.pop()
> > > Traceback (most recent call last):
> > > File "/usr/lib64/python3.10/code.py", line 90, in runcode
> > > exec(code, self.locals)
> > > File "<console>", line 1, in <module>
> > > BufferError: Existing exports of data: object cannot be re-sized
> >
> > Wow what a horrible error message! However it comes from Python, not
> > us, so there's not a lot we can do about it.
>
> Yeah, it doesn't tell you how to find which object is still hanging on
> to the buffer export. And during development, I had several times
> where I didn't call enough Py_DECREF(), which was interesting to track
> down where the stale references were being kept when all I had was
> this message to go on.
>
> >
> > > nbd> h.poll(-1)
> > > nbd> h.aio_command_completed(c)
> > > True
> > > nbd> b._o.pop()
> > > 0
> > > nbd> b.size()
> > > 9
>
> But I'm pretty happy about the results - Python's ability to magically
> lock a bytearray from being resized while a memoryview is active, so
> that we can then peer into its underlying C memory without copying,
> then restore full functionality when we're done with the underlying
> memory, is pretty cool.
>
> As to your question about ref-counting:
>
> > > +++ b/generator/Python.ml
> > > @@ -424,16 +424,12 @@ let
> > > pr " %s_view = nbd_internal_py_get_aio_view (%s, true);\n" n n;
> > > pr " if (!%s_view) goto out;\n" n;
> > > pr " py_%s = PyMemoryView_GET_BUFFER (%s_view);\n" n n;
> > > - pr " /* Increment refcount since buffer may be saved by libnbd.
> > > */\n";
> > > - pr " Py_INCREF (%s);\n" n;
> > > - pr " completion_user_data->buf = %s;\n" n
> > > + pr " completion_user_data->view = %s_view;\n" n
>
> Pre-patch and post-patch: buf_view provides a new object (reference
> count incremented to at least 1), as a result of calling
> nbd_internal_py_get_aio_view. Pre-patch did not want that reference
> count left around, so it cleans up that reference in the same function
> at [1]; rather, pre-patch wanted to save a different object (buf) and
> had to both Py_INCREF() it as well as assigning buf into
> completion_user_data for later cleanup [2]. Post-patch WANTS to use
> the reference count on view as our long-term lock, so it no longer
> needs in-function cleanup at [1], and no longer has to mess with the
> reference counts on buf.
>
> > > | BytesPersistOut (n, _) ->
> > > pr " %s_view = nbd_internal_py_get_aio_view (%s, false);\n" n n;
> > > pr " if (!%s_view) goto out;\n" n;
> > > pr " py_%s = PyMemoryView_GET_BUFFER (%s_view);\n" n n;
> > > - pr " /* Increment refcount since buffer may be saved by libnbd.
> > > */\n";
> > > - pr " Py_INCREF (%s);\n" n;
> > > - pr " completion_user_data->buf = %s;\n" n
> > > + pr " completion_user_data->view = %s_view;\n" n
> > > | Closure { cbname } ->
> > > pr " %s.user_data = %s_user_data = alloc_user_data ();\n" cbname
> > > cbname;
> > > pr " if (%s_user_data == NULL) goto out;\n" cbname;
> > > @@ -538,8 +534,7 @@ let
> > > pr " if (%s.obj)\n" n;
> > > pr " PyBuffer_Release (&%s);\n" n
> > > | BytesOut (n, _) -> pr " Py_XDECREF (%s);\n" n
> > > - | BytesPersistIn (n, _) | BytesPersistOut (n, _) ->
> > > - pr " Py_XDECREF (%s_view);\n" n
> > > + | BytesPersistIn (n, _) | BytesPersistOut (n, _) -> ()
>
> [1] mentioned above, where we cleaned up buf_view pre-patch but not
> post-patch.
>
> > > | Closure { cbname } ->
> > > pr " free_user_data (%s_user_data);\n" cbname
> > > | Enum _ -> ()
> > > @@ -588,7 +583,7 @@ let
> > > pr " */\n";
> > > pr "struct user_data {\n";
> > > pr " PyObject *fn; /* Optional pointer to Python function. */\n";
> > > - pr " PyObject *buf; /* Optional pointer to persistent buffer. */\n";
> > > + pr " PyObject *view; /* Optional PyMemoryView of persistent buffer.
> > > */\n";
> > > pr "};\n";
> > > pr "\n";
> > > pr "static struct user_data *\n";
> > > @@ -609,7 +604,7 @@ let
> > > pr "\n";
> > > pr " if (data) {\n";
> > > pr " Py_XDECREF (data->fn);\n";
> > > - pr " Py_XDECREF (data->buf);\n";
> > > + pr " Py_XDECREF (data->view);\n";
>
> [2], where we cleaned up buf pre-patch, and buf_view post-patch.
>
> > > pr " free (data);\n";
> > > pr " }\n";
> > > pr "}\n";
> >
> > I gather it works, but I don't understand why. What increments the
> > ref count on data->view?
>
> I hope that answered your question.
Sure thing, thanks.
Reviewed-by: Richard W.M. Jones <rjo...@redhat.com>
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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