Re: scratch/igc fe90d556834: Make glyphs ambiguous roots

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Helmut Eller <eller.helmut@gmail.com> writes:

> On Mon, Jul 29 2024, Gerd Möllmann wrote:
>
>>> While fix_frame runs, all other threads are suspended.
>>
>> I don't think that is guranteed. but please check.
>>
>> Frames are not roots, so fix_frame can only assume exclusive access to
>> the frame struct itself. Other threads can work on glyph_rows, glyphs
>> and so on while fix_frame runs.
>
> The doc
> https://memory-pool-system.readthedocs.io/en/latest/topic/format.html#cautions
> says:
>
>   1. The MPS guarantees that format methods have exclusive access to the
>      object for the duration of the call. This guarantee may entail
>      suspending arbitrary threads. The methods that manipulate the object
>      must not perform any sort of inter-thread locking or communication.
>   ...
>
>   6. Subject to the above constraints, format methods can freely access:
>      a. memory inside the object or block that they have been asked to
>         look at;
>      b. MPS-managed memory in pools that do not protect their contents;
>      c. memory not managed by the MPS.
>
> While the doc doesn't guarantee that all other (registered) threads are
> suspended, it's hard to imagine what else MPS could do to stop other
> threads from messing up the object.  (MPS must remove hardware barriers,
> so that's not an option.)

Good point, indeed.

I've reverted that change. Thanks for checking!

>
>>> So why don't we have exclusive access? And why is it not a problem for
>>> FRAME_FONT, FRAME_DISPLAY_INFO, etc?
>>
>> FRAME_X_DISPLAY_INFO has the same problem. It's just that I had the
>> glyphs on my todo list. With frame->font I don't see a problem. Fonts
>> are pvecs so we only have to fix the pointer to it, and fix_font should
>> do the rest.
>
> There is no frame->font.  Only a frame->output_data.ns->font.  And
> ns_output is not part of the frame struct.
>
>> BTW, unrelated. Do you observe problems in clear_face_cache?
>
> Nothing that I'm aware off.

Too bad, then it might be another macOS thing :-(.