* smob mark functions in 2.0
@ 2011-11-23 21:20 Andy Wingo
2011-11-23 23:12 ` Ludovic Courtès
2011-11-30 15:29 ` Mark H Weaver
0 siblings, 2 replies; 7+ messages in thread
From: Andy Wingo @ 2011-11-23 21:20 UTC (permalink / raw)
To: guile-user
Hello all,
I don't know how you feel, but for me the BDW garbage collector used in
Guile 2.0 has made my life much easier than it was. You typically don't
have to write SMOB marking functions any more.
However, I just fixed a bug in g-wrap that was a bit surprising to me.
If you do implement a SMOB marking function, and you touch Scheme
objects in that marking function, you need to be very careful.
Specifically, there is a warning in gc/gc_mark.h:
/* WARNING: Such a mark procedure may be invoked on an unused object */
/* residing on a free list. Such objects are cleared, except for a */
/* free list link field in the first word. Thus mark procedures may */
/* not count on the presence of a type descriptor, and must handle this */
/* case correctly somehow. */
So, your mark function might see freed objects. This is terrible, but
it is the way that it is. The key is that, if you touch a Scheme object
in your mark function, to first do a check on that object, to see that
it is valid. You can check the TC bits of the first word, or otherwise
check that other words are non-NULL.
Caveat mercator!
Andy
--
http://wingolog.org/
^ permalink raw reply [flat|nested] 7+ messages in thread* Re: smob mark functions in 2.0 2011-11-23 21:20 smob mark functions in 2.0 Andy Wingo @ 2011-11-23 23:12 ` Ludovic Courtès 2011-11-24 10:56 ` Andy Wingo 2011-11-30 15:29 ` Mark H Weaver 1 sibling, 1 reply; 7+ messages in thread From: Ludovic Courtès @ 2011-11-23 23:12 UTC (permalink / raw) To: guile-user Hello! Andy Wingo <wingo@pobox.com> skribis: > Specifically, there is a warning in gc/gc_mark.h: > > /* WARNING: Such a mark procedure may be invoked on an unused object */ > /* residing on a free list. Such objects are cleared, except for a */ > /* free list link field in the first word. Thus mark procedures may */ > /* not count on the presence of a type descriptor, and must handle this */ > /* case correctly somehow. */ Arrgh! > So, your mark function might see freed objects. This is terrible, but > it is the way that it is. The key is that, if you touch a Scheme object > in your mark function, to first do a check on that object, to see that > it is valid. You can check the TC bits of the first word, or otherwise > check that other words are non-NULL. What about making that check in libguile before invoking the user’s mark function? Thanks, Ludo’. ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: smob mark functions in 2.0 2011-11-23 23:12 ` Ludovic Courtès @ 2011-11-24 10:56 ` Andy Wingo 2011-11-24 23:24 ` Ludovic Courtès 0 siblings, 1 reply; 7+ messages in thread From: Andy Wingo @ 2011-11-24 10:56 UTC (permalink / raw) To: Ludovic Courtès; +Cc: guile-user On Thu 24 Nov 2011 00:12, ludo@gnu.org (Ludovic Courtès) writes: >> So, your mark function might see freed objects. This is terrible, but >> it is the way that it is. The key is that, if you touch a Scheme object >> in your mark function, to first do a check on that object, to see that >> it is valid. You can check the TC bits of the first word, or otherwise >> check that other words are non-NULL. > > What about making that check in libguile before invoking the user’s mark > function? Yes, we do that. I think you wrote that code! The problem was in a mark function, accessing *other* Scheme objects. Andy -- http://wingolog.org/ ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: smob mark functions in 2.0 2011-11-24 10:56 ` Andy Wingo @ 2011-11-24 23:24 ` Ludovic Courtès 0 siblings, 0 replies; 7+ messages in thread From: Ludovic Courtès @ 2011-11-24 23:24 UTC (permalink / raw) To: Andy Wingo; +Cc: guile-user Hi, Andy Wingo <wingo@pobox.com> skribis: > On Thu 24 Nov 2011 00:12, ludo@gnu.org (Ludovic Courtès) writes: > >>> So, your mark function might see freed objects. This is terrible, but >>> it is the way that it is. The key is that, if you touch a Scheme object >>> in your mark function, to first do a check on that object, to see that >>> it is valid. You can check the TC bits of the first word, or otherwise >>> check that other words are non-NULL. >> >> What about making that check in libguile before invoking the user’s mark >> function? > > Yes, we do that. I think you wrote that code! Yes, when I was young. ;-) > The problem was in a mark function, accessing *other* Scheme objects. Oh, right. Anyway, users are encouraged to #ifdef out mark functions when using Guile 2.0. Thanks, Ludo’. ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: smob mark functions in 2.0 2011-11-23 21:20 smob mark functions in 2.0 Andy Wingo 2011-11-23 23:12 ` Ludovic Courtès @ 2011-11-30 15:29 ` Mark H Weaver 2014-09-26 4:50 ` Mark H Weaver 1 sibling, 1 reply; 7+ messages in thread From: Mark H Weaver @ 2011-11-30 15:29 UTC (permalink / raw) To: Andy Wingo; +Cc: guile-user Andy Wingo <wingo@pobox.com> writes: > Specifically, there is a warning in gc/gc_mark.h: > > /* WARNING: Such a mark procedure may be invoked on an unused object */ > /* residing on a free list. Such objects are cleared, except for a */ > /* free list link field in the first word. Thus mark procedures may */ > /* not count on the presence of a type descriptor, and must handle this */ > /* case correctly somehow. */ > > So, your mark function might see freed objects. How can this happen? If you are marking an object, then presumably it is still reachable, and therefore the objects it references are also still reachable. If any of those reachable objects has been freed, isn't that already a bug of a different kind? What am I missing? Thanks, Mark ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: smob mark functions in 2.0 2011-11-30 15:29 ` Mark H Weaver @ 2014-09-26 4:50 ` Mark H Weaver 2014-09-27 15:29 ` Andy Wingo 0 siblings, 1 reply; 7+ messages in thread From: Mark H Weaver @ 2014-09-26 4:50 UTC (permalink / raw) To: Andy Wingo; +Cc: guile-user Hi Andy, Reviving a 3-year-old thread... Andy Wingo <wingo@pobox.com> wrote in November 2011: > If you do implement a SMOB marking function, and you touch Scheme > objects in that marking function, you need to be very careful. > > Specifically, there is a warning in gc/gc_mark.h: > > /* WARNING: Such a mark procedure may be invoked on an unused object */ > /* residing on a free list. Such objects are cleared, except for a */ > /* free list link field in the first word. Thus mark procedures may */ > /* not count on the presence of a type descriptor, and must handle this */ > /* case correctly somehow. */ > > So, your mark function might see freed objects. This is terrible, but > it is the way that it is. The key is that, if you touch a Scheme object > in your mark function, to first do a check on that object, to see that > it is valid. You can check the TC bits of the first word, or otherwise > check that other words are non-NULL. Andy Wingo <wingo@pobox.com> later replied to Ludovic: > On Thu 24 Nov 2011 00:12, ludo@gnu.org (Ludovic Courtès) writes: > >>> So, your mark function might see freed objects. This is terrible, but >>> it is the way that it is. The key is that, if you touch a Scheme object >>> in your mark function, to first do a check on that object, to see that >>> it is valid. You can check the TC bits of the first word, or otherwise >>> check that other words are non-NULL. >> >> What about making that check in libguile before invoking the user’s mark >> function? > > Yes, we do that. I think you wrote that code! The problem was in a > mark function, accessing *other* Scheme objects. Mark H Weaver <mhw@netris.org> writes: > Andy Wingo <wingo@pobox.com> writes: >> Specifically, there is a warning in gc/gc_mark.h: >> >> /* WARNING: Such a mark procedure may be invoked on an unused object */ >> /* residing on a free list. Such objects are cleared, except for a */ >> /* free list link field in the first word. Thus mark procedures may */ >> /* not count on the presence of a type descriptor, and must handle this */ >> /* case correctly somehow. */ >> >> So, your mark function might see freed objects. > > How can this happen? If you are marking an object, then presumably it > is still reachable, and therefore the objects it references are also > still reachable. If any of those reachable objects has been freed, > isn't that already a bug of a different kind? What am I missing? I never received an answer to this question. At the time it was merely a curiosity, but now I have a more pressing need to understand what's going on here. Regards, Mark ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: smob mark functions in 2.0 2014-09-26 4:50 ` Mark H Weaver @ 2014-09-27 15:29 ` Andy Wingo 0 siblings, 0 replies; 7+ messages in thread From: Andy Wingo @ 2014-09-27 15:29 UTC (permalink / raw) To: Mark H Weaver; +Cc: guile-user On Fri 26 Sep 2014 06:50, Mark H Weaver <mhw@netris.org> writes: > Mark H Weaver <mhw@netris.org> writes: > >> Andy Wingo <wingo@pobox.com> writes: >>> Specifically, there is a warning in gc/gc_mark.h: >>> >>> /* WARNING: Such a mark procedure may be invoked on an unused object */ >>> /* residing on a free list. Such objects are cleared, except for a */ >>> /* free list link field in the first word. Thus mark procedures may */ >>> /* not count on the presence of a type descriptor, and must handle this */ >>> /* case correctly somehow. */ >>> >>> So, your mark function might see freed objects. >> >> How can this happen? This can happen if the object is on a free list. Free lists are just normal heap-managed data structures -- traced by the GC, counting as live. See e.g. gc-inline.h in master; a variation of this exact code is used internally in libgc by the allocation API. Andy -- http://wingolog.org/ ^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2014-09-27 15:29 UTC | newest] Thread overview: 7+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2011-11-23 21:20 smob mark functions in 2.0 Andy Wingo 2011-11-23 23:12 ` Ludovic Courtès 2011-11-24 10:56 ` Andy Wingo 2011-11-24 23:24 ` Ludovic Courtès 2011-11-30 15:29 ` Mark H Weaver 2014-09-26 4:50 ` Mark H Weaver 2014-09-27 15:29 ` Andy Wingo
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