smob GC question

[Eric Enderton]

i'm reading data-rep.info, particularly the section "A Common
Mistake In Allocating Smobs".  (it's great -- thank you!)

i have one question/concern, right at the end:

        A correct implementation might say, instead:
           image->name = SCM_BOOL_F;
           image->update_func = SCM_BOOL_F;
         
           SCM_NEWCELL (image_smob);
           SCM_SETCDR (image_smob, image);
           SCM_SETCAR (image_smob, image_tag);
         
           image->name = scm_string_copy (name);
           image->update_func = scm_make_gsubr (...);
         
           return image_smob;
        
        Now, by the time we allocate the new string and function objects,
        `image_smob' points to `image'.  If the garbage collector scans the
        stack, it will find a reference to `image_smob' and traverse `image',
        so any objects `image' points to will be preserved.

if GC runs during, say, the scm_make_gsubr(), where does it
find a reference to image_smob?  no cells point to it yet.
and if it doesn't get marked, aren't we in trouble?

thanks for any enlightenment,
-E

p.s.  i sure hope you only get one copy of this ...