Eli Zaretskii <eliz@gnu.org> writes:

>> > Anyway, are you saying that stack marking doesn't work in optimized
>> > code?  We've been using this technique for the last 17 years without
>> > problems; why would the fact that we have more than one thread change
>> > that?  The same arguments you submit are valid for a single-threaded
>> > Emacs, right?
>> 
>> Apparently so.  I set up a single-threaded situation where I could
>> redefine a function while exec_byte_code was running it, and got a
>> segfault.  I've gained some insights from debugging this version of the
>> bug which I will put into a separate email.
>
> If this is the case, then I think we should protect the definition of
> a running function from GC, in some way, either by making sure it is
> referenced by some stack-based Lisp object, even in heavily optimized
> code (e.g., by using 'volatile' qualifiers); or by some other method
> that will ensure that definition is marked and not swept.

Maybe code optimizers have improved over the last 17 years?

I have patched Emacs with a 'volatile' on the definition of 'fun' in
Ffuncall, and so far haven't managed to reproduce the bug with it: