bug#17168: 24.3.50; Segfault at mark_object

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 06 Apr 2014 09:37:23 -0700
> From: Daniel Colascione <dancol@dancol.org>
> CC: monnier@IRO.UMontreal.CA, dmantipov@yandex.ru, 17168@debbugs.gnu.org
> 
> > Because Richard has been using that machine for years, and I very much
> > doubt that he changed his usage patterns lately.
> 
> Richard's not the only one who has seen this crash. Drew's also reported
> GC crashes in odd, and different, places.

Which seem unrelated, and started much later than Richard reported
his.

> >>>>> In http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15583#23, Richard
> >>>>> provided the last good revno (113938) and the first bad one (114268);
> >>>>> I looked at that range of revisions, and 114156 looks relevant.  How
> >>>>> about if we revert it and see if the problems go away?
> >>>>
> >>>> The bug would still be there, and we'd have no way to tell whether your
> >>>> proposed change actually reduced its occurrence to a tolerable level.
> >>>> Why would you want to do that instead of just fixing the bug?
> >>>
> >>> Because it's simpler,
> >>
> >> It's easy to make code that's simple and wrong.
> > 
> > I didn't suggest any new code.
> 
> No: you're just suggesting leaving incorrect code in Emacs.

It's not incorrect, AFAIU.  It might be less optimal.

> >>> and because it just might be that the bug was
> >>> caused by that other changeset.
> >>
> >> How might that changeset in particular have caused the problem reports?
> > 
> > It is related to calling a function, and is in the same function from
> > which all the recent crashes started.
> 
> You haven't identified a causal mechanism. Any recent change could have
> caused enough of a shift in code generation or stack layout to cause
> this problem, and because it manifests so seldom, it'd be hard to verify
> that reverting any particular change "fixed" the problem.

I thought you had a test case.  If not, how did you verify that your
suggested changes do fix the problem?

> Also, eval_sub does *everything*. It's no surprise that we saw the
> crashes there. That's like saying "all crashes are associated with main,
> this change affects main, and therefore this change is responsible."

The change is related to calling a function whose symbol has certain
properties.  That sounds related to me, not just a random change
somewhere in eval_sub.

Anyway, it was just an idea which I thought would be easy to try.