Re: possible gc/weak hash table bug

[michaelawells]

I replaced vacuum_weak_hash_table with the following:

> static void
> vacuum_weak_hash_table (SCM table)
> {
>   SCM buckets = SCM_HASHTABLE_VECTOR (table);
>   unsigned long k = SCM_SIMPLE_VECTOR_LENGTH (buckets);
>   size_t len = SCM_HASHTABLE_N_ITEMS (table);
> 
>   unsigned long org_k = SCM_SIMPLE_VECTOR_LENGTH (buckets);
>   size_t org_len = SCM_HASHTABLE_N_ITEMS (table);
> 
> 
>   if (len>0) {
>   while (k--)
>     {
>       size_t removed;
>       SCM alist = SCM_SIMPLE_VECTOR_REF (buckets, k);
>       alist = scm_fixup_weak_alist (alist, &removed);
>       if (! (removed <= len)) {
>       fprintf(stderr,"$$$ k = %lu (%lu) removed = %i   len = %i 
> (%i)\n",k,org_k,removed,len,org_len);
>       }
>       assert (removed <= len);
>       len -= removed;
>       SCM_SIMPLE_VECTOR_SET (buckets, k, alist);
>     }
>   
>   SCM_SET_HASHTABLE_N_ITEMS (table, len);
>   }
> }

Two failure occurred in two different runs.  Each time, the debugging message 
was the same:

> $$$ k = 89 (113) removed = 1   len = 0 (5)
> hashtab.c:145: vacuum_weak_hash_table: Assertion `removed <= len' failed.

This happened while vacuuming bucket 89 (of 113).  The original length was 5.

Again, this problem occurs both infrequently and intermittently.  

On Mar 17, 2011, at 7:09 PM, michaelawells wrote:

> As requested, I'm posting this to address@hidden
> 
> I'm now seeing this assertion failure, using a guile development snapshot 
> (guile-2.0.0.104-f5fc7):
> 
>> dpm.linux: hashtab.c:136: vacuum_weak_hash_table: Assertion `removed <= len' 
>> failed.
> 
> 
>> static void
>> vacuum_weak_hash_table (SCM table)
>> {
>>  SCM buckets = SCM_HASHTABLE_VECTOR (table);
>>  unsigned long k = SCM_SIMPLE_VECTOR_LENGTH (buckets);
>>  size_t len = SCM_HASHTABLE_N_ITEMS (table);
>> 
>>  while (k--)
>>    {
>>      size_t removed;
>>      SCM alist = SCM_SIMPLE_VECTOR_REF (buckets, k);
>>      alist = scm_fixup_weak_alist (alist, &removed);  <<<**** FAILS HERE
>>      assert (removed <= len);
>>      len -= removed;
>>      SCM_SIMPLE_VECTOR_SET (buckets, k, alist);
>>    }
>> 
>>  SCM_SET_HASHTABLE_N_ITEMS (table, len);
>> }
>> 
> 
> 
> Unfortunately, this problem is infrequent and intermittent.  In my case, it 
> happened in 10 out of 1000 runs.  
> 
> Currently, I don't know of a way to reproduce the problem.
> 
> 
> 
> On Mar 17, 2011, at 4:43 AM, Andy Wingo wrote:
> 
>> Hi Michael,
>> 
>> Thanks for the report.  In the future, please copy address@hidden; it
>> doesn't hurt.
>> 
>> On Thu 17 Mar 2011 00:56, michaelawells <address@hidden> writes:
>> 
>>> hashtab.c:203: weak_bucket_assoc: Assertion `(((scm_t_hashtable *)
>>> ((((scm_t_bits) (0? (*(SCM*)0=((((SCM *)((scm_t_cell *) (((scm_t_bits)
>>> (0? (*(SCM*)0=((((table))))): (((table)))))))) [((2))]))): (((SCM
>>> *)((scm_t_cell *) (((scm_t_bits) (0? (*(SCM*)0=((((table))))):
>>> (((table)))))))) [((2))]))))))->n_items) >= args.removed_items'
>>> failed.
>>> 
>>> Did you ever see this assertion fail?  
>> 
>> No I didn't, and that bothers me a little.
>> 
>>> I tried using a development snapshot (guile-2.0.0.104-f5fc7).  When I
>>> use this snapshot, I do not see this problem.
>> 
>> There were certainly some bugs fixed.  Let's assume that it's fixed in
>> git, and in the upcoming 2.0.1 (should be a week or so).  There are
>> still some lingering issues with weak hash tables, but we are working on
>> them; and certainly no crashes that we know of.
>> 
>> Have fun with Guile,
>> 
>> Andy
>> -- 
>> http://wingolog.org/
>