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Re: GC bug investigation

From: Richard Stallman
Subject: Re: GC bug investigation
Date: Sun, 23 Mar 2014 10:57:34 -0400

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    Details of the objects on the path might be useful.

I don't understand "on the path".



    B here is clear-transient-map's function cell, right? You're saying you
    saw that it's a pseudovector that safe_debug_print reports as
    INVALID_LISP_OBJECT, probably because live_vector_p returns 0.


    we're reaching B at all indicates that it shouldn't be dead.

I guess so.  This is the mysterious part.

    B must have been made dead *before* being assigned to
    clear-transient-map's function cell. Looking at the bytecode in
    set-transient-map, though, I don't see how that's possible.

I don't think that's what happened.  If it were that, we would
see crashes when that code tries to _use_ the value legitimately.

    clear-transient-map isn't dead either,

It has not been freed, it seems, but it may be garbage.

It is being marked through a spurious pointer randomly hanging around
in a stack slot for something else.  We don't know that there is any
real pointer to it.

    I don't think that writing code that aborts or breaks when a particular
    vector is freed will be very helpful; we'll hit that code in normal
    operation too. Instead, it'll probably be more useful to print a
    backtrace (using emacs_backtrace) each time we see that vectorlike
    freed, then look at the last backtrace before the GC crash.

Maybe you are right.

    Can you try running with -DGC_CHECK_MARKED_OBJECTS=1 in your CFLAGS?

I can, but it would be a big pain.  It takes many hours to recompile
Emacs on this machine.

What would it tell us?  It would confirm that the vectorlike was freed,
perhaps, but do we doubt that?

If that hassle is likely to solve the problem, I'll do it,
but I'd rather not go to that hassle just to confirm what we know.

Dr Richard Stallman
President, Free Software Foundation
51 Franklin St
Boston MA 02110
www.fsf.org  www.gnu.org
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