gc crash caused by improper use of GC_general_register_disappearing_link

Fri Nov 12 20:03:00 GMT 2004

Boehm, Hans writes:
 > The GC interface is a bit inconsistent here. I changed the
 > finalizer registration interface to explicitly disregard requests
 > to register a finalizer for static data (gcj does that, too),
 > but I didn't change the disappearing link one.
 > 
 > The way the code currently works, it is cheap to do this in the
 > finalizer case (a null test after a block header lookup), but requires
 > extra work in the disappearing link case, since the header lookup
 > isn't otherwise being performed. But it might independently
 > make sense to do the header lookup in both cases, and cache the
 > header address.
 > 
 > Thus I could go either way on having the GC or libgcj do this
 > check. (It seems equally clean either way. Since static objects
 > don't become unreachable, these calls should logically be no-ops.
 > Having the two work inconsistently is not ideal, though.)
 > 
 > A quick fix would be to have libgcj check GC_base(<pointer>)!=0
 > before registering the pointer. A faster but uglier way is to
 > use GC_find_header() instead of GC_base().
OK, I'll do that.
 > We could use GC_find_header as a stopgap, and have GC7 do the
 > right thing.
Good.
Thanks,
Andrew.
 > > -----Original Message-----
 > > From: Andrew Haley [mailto:aph@redhat.com]
 > > Sent: Friday, November 12, 2004 10:34 AM
 > > To: Boehm, Hans
 > > Cc: java@gcc.gnu.org
 > > Subject: gc crash caused by improper use of
 > > GC_general_register_disappearing_link
 > > 
 > > 
 > > I had a crash inside the gc. It turns out that someone made a weak
 > > reference to an instance of class Class. However, some instances of
 > > Class are statically allocated, and the doc for
 > > GC_general_register_disappearing_link says
 > > 
 > > "Obj must be a pointer to the first word of an object we allocated."
 > > 
 > > So, this call to GC_general_register_disappearing_link is illegal. 
 > > 
 > > Is there some reasonably easy way we can do something like
 > > 
 > > if (thing points to something that is gc allocated)
 > > GC_general_register_disappearing_link(thing)
 > > 
 > > We intend to heap allocate instances of Class in the future, but I'd
 > > prefer not to have to do so right now.
 > > 
 > > Thanks,
 > > Andrew.
 > >