Re: Some experience with the igc branch

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Pip Cet <pipcet@protonmail.com> writes:
>>
>>> I'll spare you most of the details for now, but having read the mps
>>> header, MPS allocation is not safe to use from separate threads without
>>> locking the AP (or having per-thread APs), which we might end up doing
>>> on Windows, IIRC.
>>
>> Now I'm confused. We're using thread allocation points. See
>> create_thread_aps, thread_ap, and so on.
>
> I was confused.  This is only a problem if we allocate memory from a
> signal handler, which is effectively sharing the per-thread structure.
>
> (I'm still confused. My patch worked on the first attempt, which my code
> never does.  I suspect that while I made a mistake, it caused a subtle
> bug rather than an obvious one.)
>
> And we don't want to allocate memory from signal handlers, right?  We
> could, now (see warnings below):

Can't speak for others, but I wouldn't want it :-).

I can't cite myself, but I'm pretty sure I said some time ago already
that in a portable program one cannot do much in a signal handler in the
first place. So I wouldn't be surprised if MPS didn't support being
called from a signal handler. Not unreasonable for me.

But whatever. Maybe Richard Brooksby answers, and can shed light on that
or has ideas, if we don't overload him :-). And anyway, there is now
something workable in the igc branch. Maybe we could wait a bit, and
just proceed with something else meanwhile.

[... Thanks for the patch ...]

> Warnings:
>
> This is the "slow path" only, used for all allocations. Will cause a
> great number of busy-looping threads.  

Don't know why, but the busy looping threads makes me feel a bit
uncomfortable :-),

> Will be very slow. Creating additional emacs threads will result in a
> proportional number of additional threads, which will be very, very
> slow, so don't. Requires pthread.h and stdatomic.h, and still does
> things not covered by those APIs (memcpying over an atomic_uintptr_t,
> even if we know that its value won't change, is probably verboten, and
> definitely should be). I *think* this code might work if we allocate
> from signal handlers, and I think this code might work on systems that
> don't have lock-free atomics (once the #error is removed), but it
> definitely won't do both at the same time.
>
> Pip

BTW, do you know which signal handlers use Lisp, i.e. allocate Lisp
objects or access some? All? Or, would it be realistic to rewrite signal
handlers to not do that?

One thing I've seen done elsewhere is to publish a message to a message
board so that it can be handled outside of the signal handler. Something
like that, you know what I mean.