Re: MPS: a random backtrace while toying with gdb

[Pip Cet <pipcet@protonmail.com>]

On Sunday, June 30th, 2024 at 10:34, Eli Zaretskii <eliz@gnu.org> wrote:
> > Date: Sun, 30 Jun 2024 10:16:42 +0000
> > From: Pip Cet pipcet@protonmail.com
> > Cc: gerd.moellmann@gmail.com, yantar92@posteo.net, emacs-devel@gnu.org, eller.helmut@gmail.com
> >
> > > > We could always "steal" the SIGSEGV handler and reinstall it with a sigmask (without modifying the MPS source). My guess is that's equivalent to what the macOS code does, essentially, by using a different class of signal that blocks POSIX signals...
> > >
> > > And this is less "ugly" (let alone more safe) than using sigblock?
> > > What am I missing?

And, yes, as far as I can tell my proposed solution is safer.

> > Mostly, I think, the race condition where the barrier is installed (by MPS) some time before dflt_scan is even called (and, symmetrically, we'd unblock it too early while the barrier is still in effect). IOW, sigblock wouldn't work.
>
>
> Then we should block signals earlier, e.g., in igc_alloc (and any
> other place which could cause our objects moved).

That would be a very expensive and error-prone way of achieving the same result as simply blocking the signals while handling SIGSEGV.

> > (I should point out that you've convinced me as far as SIGPROF goes. At least for the initial stage, it makes more sense to count hits in GC (which we do, with false positives, using the current scratch/igc code) than to delay signal processing until GC is over).
>
>
> SIGPROF is not the only one.

It's the only one where I am not convinced the right thing is to simply block it using sa_sigmask.

> We have other useful signals, and we
> need to think about a solution that will cover most if not all of
> them. I've listed the signals that came to mind a few messages ago.

Those would be handled just fine by blocking all but the known-to-be-safe SIGPROF while handling SIGSEGV.

> I still cannot believe MPS doesn't have a canonical solution for these
> cases.

I haven't been able to find one in the docs. It still seems a simple oversight in the MPS code to me that they don't simply block all signals while handling SIGSEGV, or provide an API to set the mask.

Pip