Re: Congratulation! Bug found!! (GC & C interfacing problems)

Re: Congratulation! Bug found!! (GC & C interfacing problems)

[Damien Doligez]

>From: David.Mentre@irisa.fr (David=?iso-8859-1?q?_Mentr=E9?=)

>I also subscribe to this documentation revision. I also volunteer, if
>needed, to review/rewrite the doc part related to Interfacing C with
>OCaml.

We are always very grateful for any contribution to the system and
especially the docs.  I've added a few notes in the doc based on your
story (as found in my mailbox when I came back from vacation), but I
guess what we really need is a better example to show all the features
of the interface.

-- Damien

Re: Congratulation! Bug found!! (GC & C interfacing problems)

[Markus Mottl]

>   3. However, this macro is calling modify (function defined in
>      byterun/memory.c) which in turn calls the Modify macro (defined in
>      byterun/memory.h). As Markus said, this macro adds the address
>      given in argument to a list of memory addresses (ref_table_ptr)
>      that should be examined by the GC at collection time.
[...]
> I also subscribe to this documentation revision. I also volunteer, if
> needed, to review/rewrite the doc part related to Interfacing C with
> OCaml.

I guess that the confusion about the real things happening as explained in
"3" above comes from "rule 6" in the documentation of the C-interface,
which says:

  Direct assignment to a field of a block, as in

          Field(v, n) = w;

  is safe only if v is a block newly allocated by alloc_small; that
  is, if no allocation took place between the allocation of v and the
  assignment to the field. In all other cases, never assign directly.

This "safe only" and "in all other cases, never assign directly" leaves the
impression that the "Field" macro is a bit "evil" and could be avoided,
possibly by using this nice "Store_field"-macro. However, it is not only
"safe" to use "Field" in this case, it seems (?) that this is the only way
to do it correctly. Furthermore, I did not find any documentation on
correctly placing values into blocks created with "alloc_final", which
seems to be pretty similar to "alloc_small" in this respect.

The only information I found concerning "alloc_small" which appears to
indicate correct usage is:

  alloc_small(n, t) returns a fresh small block of size n <=
  Max_young_wosize words, with tag t. If this block is a structured block
  (i.e. if t < No_scan_tag), then the fields of the block (initially
  containing garbage) must be initialized with legal values (using direct
  assignment to the fields of the block) before the next allocation.

The intention of "using direct assignment to the fields" is obviously meant
as hint to use the "Field"-macro. Because most people don't know that
"Store_field" not only assigns directly, but does unexpected other things,
this information does probably not help...

Best regards,
Markus Mottl

-- 
Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl

Congratulation! Bug found!! (GC & C interfacing problems)

[David Mentré]

Hi Markus, Hi all camlists,

You were right Markus. Using directly the Field macro fixed my bug.

Markus Mottl <mottl@miss.wu-wien.ac.at> writes:

> You use "Store_field" throughout the code to assign pointers to fields in
> structures which were allocated using "alloc_final".
> 
> I once had a similar bug in my PCRE-library, but Gerd Stolpmann was so kind
> to send me the patch and explain the problem. Here his translated
> explanation (seems reasonable):
> 
>   - after "alloc_small" the fields have to be initialized with
>     "Field(var, n) = ...", not with "Store_field". The last version writes
>     (with some bad luck) the address of the field into a list of addresses
>     which have to be moved in case of a minor GC.
> 
>   - The fields of "alloc_final" are not considered by the GC. Therefore,
>     they, too, have to be written to using "Field(var, n)" (or you may
>     cast them to a normal C-struct). "Store_field" has, again, unexpected
>     side effects.

The explanation (or a guess ;) :

  1. a memory block is allocated with alloc_final, therefore this block
     internals should not be considered by the GC.

  2. I use the Store_field macro to update block content. 

  3. However, this macro is calling modify (function defined in
     byterun/memory.c) which in turn calls the Modify macro (defined in
     byterun/memory.h). As Markus said, this macro adds the address
     given in argument to a list of memory addresses (ref_table_ptr)
     that should be examined by the GC at collection time.

  4. So, we have a GC-opaque memory block whose content adresses have
     been added to a GC to-examine-later list. Therefore, at GC time:
     crash.


> In case this is really the bug (probably), I'd suggest a revision of the
> C-interface-documentation. At least to me it was not obvious that
> "Store_field" leads to such additional, unexpected behaviour.

I also subscribe to this documentation revision. I also volunteer, if
needed, to review/rewrite the doc part related to Interfacing C with
OCaml.


> Good luck squeezing the bug,

I've squeezed it, with your help. :)

One again, many many thanks,
Best regards,
david
-- 
 David.Mentre@irisa.fr -- http://www.irisa.fr/prive/dmentre/
 Opinions expressed here are only mine.