[Gc] GC_INIT? Threads?

Bruce Hoult bruce at hoult.org
Tue Aug 20 06:37:45 PDT 2013


It would be helpful to know where it hangs. Do you not have access to a
debugger? It's also almost impossible to help without seeing the full
source code for a program that shows the problem. There are an almost
infinite number of things that *could* be done incorrectly.

For example, do you know whether GC_get_stack_base() is returning? Are you
checking its return value for GC_SUCCESS? Does GC_register_my_thread()
return? Does it return GC_SUCCESS?

If you are not using IA64 then this should be pretty foolproof:

void foo_thread(){
  struct GC_stack_base my_sb;
  my_sb.mem_base = &my_sb;
  GC_register_my_thread(&my_sb);
  foo_thread_inner();
  GC_unregister_my_thread();
}

void foo_thread_inner(){
   // do the real work here
}



On Wed, Aug 21, 2013 at 1:10 AM, The Devils Jester <
thedevilsjester at gmail.com> wrote:

> I do not know what you mean about the error.
>
> In the first scenario, without the register threads functions, it outputs
> the error I posted above :
> "Collecting from unknown thread
> Abort trap: 6"
>
> In the second scenario, with the additional functions, it enters an
> endless loop and never (within the few minutes I let it sit for) errors.
>
> I only include gc_cpp.h, and only once.  I have #define GC_THREADS
> directly before the include.
>
> Is there any additional information I can provide?
>
> Must I use GC_call_with_stack_base()?  If so, how is this function used?
>
> Is there a way I can tell if the GC was built without GC_THREADS support?
>  Wouldnt it be missing the above mentioned thread symbols from the library
> if this were the case?
>
>
>
> On Tue, Aug 20, 2013 at 1:22 AM, Bruce Hoult <bruce at hoult.org> wrote:
>
>> It's really very hard to tell without knowing what error the GC is
>> detecting and aborting on (it writes the message to stderr), and without
>> being able to see your source code.
>>
>> It could be as basic as your GC library not having been built to support
>> threads, or your client program not having done "#define GC_THREADS"
>> before including gc.h.
>>
>> You can define the GC_LOOP_ON_ABORT environment variable, in which case
>> GC_abort() will sit in an infinite loop so you can attach to the process
>> and debug it. But then you could also have just set a breakpoint in
>> GC_abort() to achieve the same end.
>>
>>
>> On Tue, Aug 20, 2013 at 6:08 PM, The Devils Jester <
>> thedevilsjester at gmail.com> wrote:
>>
>>> I apologize, I do not mean "crash" in the literal sense.  I appreciate
>>> the help finding a solution to my problem.
>>>
>>> The commands supplied, causes the program to lock up and consume all of
>>> the CPU.
>>>
>>> The code I have is as follows.  I am unsure about the stack_base or how
>>> it is used.
>>>
>>> In my primary thread:
>>>     GC_INIT();
>>>     GC_allow_register_threads();
>>>     ClassFromMyLib * myclass = new ClassFromMyLib();
>>>
>>> In my new thread:
>>>     struct GC_stack_base *my_sb;
>>>     GC_get_stack_base(my_sb);
>>>     GC_register_my_thread(my_sb);
>>>            myclass->myfunction();
>>>     GC_unregister_my_thread();
>>>
>>> Without the special GC_ functions, everything works fine until GC tries
>>> to reclaim memory, with them it locks up.
>>>
>>> Am I doing something wrong?
>>>
>>>
>>>
>>>
>>> On Tue, Aug 20, 2013 at 12:00 AM, Bruce Hoult <bruce at hoult.org> wrote:
>>>
>>>> Once again, it's not crashing, it's a controlled exit because it
>>>> detected something wrong.
>>>>
>>>> Try using these (from gc.h)
>>>>
>>>> call GC_allow_register_threads() from your main program, after GC_INIT()
>>>> call GC_register_my_thread() at the start of the main function for each
>>>> thread you create. (and GC_unregister_my_thread() at the end)
>>>>
>>>> You may also find  GC_call_with_stack_base() useful.
>>>>
>>>>   /* Explicitly enable GC_register_my_thread() invocation.
>>>>  */
>>>>   /* Done implicitly if a GC thread-creation function is called (or
>>>> */
>>>>   /* implicit thread registration is activated).  Otherwise, it must
>>>>  */
>>>>   /* be called from the main (or any previously registered) thread
>>>>  */
>>>>   /* between the collector initialization and the first explicit
>>>>  */
>>>>   /* registering of a thread (it should be called as late as possible).
>>>> */
>>>>   GC_API void GC_CALL GC_allow_register_threads(void);
>>>>
>>>>   /* Register the current thread, with the indicated stack base, as
>>>> */
>>>>   /* a new thread whose stack(s) should be traced by the GC.  If it
>>>> */
>>>>   /* is not implicitly called by the GC, this must be called before a
>>>> */
>>>>   /* thread can allocate garbage collected memory, or assign pointers
>>>> */
>>>>   /* to the garbage collected heap.  Once registered, a thread will be
>>>>  */
>>>>   /* stopped during garbage collections.
>>>>  */
>>>>   /* This call must be previously enabled (see above).
>>>>  */
>>>>   /* This should never be called from the main thread, where it is
>>>>  */
>>>>   /* always done implicitly.  This is normally done implicitly if GC_
>>>> */
>>>>   /* functions are called to create the thread, e.g. by including gc.h
>>>>  */
>>>>   /* (which redefines some system functions) before calling the system
>>>>  */
>>>>   /* thread creation function.  Nonetheless, thread cleanup routines
>>>>  */
>>>>   /* (eg., pthread key destructor) typically require manual thread
>>>>  */
>>>>   /* registering (and unregistering) if pointers to GC-allocated
>>>>  */
>>>>   /* objects are manipulated inside.
>>>>  */
>>>>   /* It is also always done implicitly on some platforms if
>>>> */
>>>>   /* GC_use_threads_discovery() is called at start-up.  Except for the
>>>>  */
>>>>   /* latter case, the explicit call is normally required for threads
>>>>  */
>>>>   /* created by third-party libraries.
>>>>  */
>>>>   /* A manually registered thread requires manual unregistering.
>>>>  */
>>>>   GC_API int GC_CALL GC_register_my_thread(const struct GC_stack_base
>>>> *);
>>>>
>>>>   /* Unregister the current thread.  Only an explicitly registered
>>>>  */
>>>>   /* thread (i.e. for which GC_register_my_thread() returns GC_SUCCESS)
>>>> */
>>>>   /* is allowed (and required) to call this function.  (As a special
>>>>  */
>>>>   /* exception, it is also allowed to once unregister the main thread.)
>>>> */
>>>>   /* The thread may no longer allocate garbage collected memory or
>>>>  */
>>>>   /* manipulate pointers to the garbage collected heap after making
>>>> */
>>>>   /* this call.  Specifically, if it wants to return or otherwise
>>>> */
>>>>   /* communicate a pointer to the garbage-collected heap to another
>>>> */
>>>>   /* thread, it must do this before calling GC_unregister_my_thread,
>>>>  */
>>>>   /* most probably by saving it in a global data structure.  Must not
>>>> */
>>>>   /* be called inside a GC callback function (except for
>>>>  */
>>>>   /* GC_call_with_stack_base() one).
>>>>  */
>>>>   GC_API int GC_CALL GC_unregister_my_thread(void);
>>>>
>>>>
>>>>
>>>> On Tue, Aug 20, 2013 at 4:48 PM, The Devils Jester <
>>>> thedevilsjester at gmail.com> wrote:
>>>>
>>>>> The only output I get, aside from the stack trace is:
>>>>> Collecting from unknown thread
>>>>> Abort trap: 6
>>>>>
>>>>> My application is organized as follows:
>>>>>
>>>>> MAIN APP (does not use GC)
>>>>>    GC_INIT()
>>>>>    NEW THREAD
>>>>>          CALL LIBRARY FUNCTION (library uses GC)
>>>>>
>>>>> This crashes, while
>>>>>
>>>>> MAIN APP (does not use GC)
>>>>>    GC_INIT()
>>>>>    CALL LIBRARY FUNCTION (library uses GC)
>>>>>
>>>>> Works perfectly fine.
>>>>>
>>>>> What am I doing wrong?  Do I have to initialize the GC in some special
>>>>> fashion?
>>>>>
>>>>>
>>>>> On Mon, Aug 19, 2013 at 11:35 PM, Bruce Hoult <bruce at hoult.org> wrote:
>>>>>
>>>>>> That's not a crash, it's a deliberate abort because some
>>>>>> sanity-checking code found that your world was insane. There are several
>>>>>> things that can go wrong inside GC_push_all_stacks, so it would be
>>>>>> helpful to have the message that GC_abort() printed.
>>>>>>
>>>>>> However, you should always call GC_INIT() from your main program, and
>>>>>> as early as possible.
>>>>>>
>>>>>> On some versions and platforms GC_INIT() installs code that registers
>>>>>> threads in a GC data structure because it's not possible to ask the OS for
>>>>>> all your threads. If you call GC_INIT() from a new thread on those
>>>>>> platforms then that thread will remain unregistered, which is one of the
>>>>>> things that will make GC_push_all_stacks abort later.
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, Aug 20, 2013 at 4:10 PM, The Devils Jester <
>>>>>> thedevilsjester at gmail.com> wrote:
>>>>>>
>>>>>>> I have a library that makes heavy use of of the GC, and it works
>>>>>>> quite well when everything (the app, and the library) is in one thread.  I
>>>>>>> call GC_INIT in the app, and all is well.
>>>>>>>
>>>>>>> If, however, the app creates a new thread that calls the library
>>>>>>> function, then it crashes when the GC does its magic (below I have pasted
>>>>>>> some relevant output).
>>>>>>>
>>>>>>> Is there some trick to using GC on a separate thread?  Do I call
>>>>>>> GC_INIT from the main thread, or the one that calls the library functions?
>>>>>>>
>>>>>>>
>>>>>>> 0   libsystem_kernel.dylib         0x00007fff8ad14212
>>>>>>> __pthread_kill + 10
>>>>>>> 1   libsystem_c.dylib             0x00007fff90d01b54 pthread_kill +
>>>>>>> 90
>>>>>>> 2   libsystem_c.dylib             0x00007fff90d45dce abort + 143
>>>>>>> 3   libgc.1.dylib                 0x000000010357676e GC_abort + 97
>>>>>>> 4   libgc.1.dylib                 0x000000010357b413
>>>>>>> GC_push_all_stacks + 285
>>>>>>> 5   libgc.1.dylib                 0x000000010357396e GC_mark_some +
>>>>>>> 377
>>>>>>> 6   libgc.1.dylib                 0x000000010356d73e
>>>>>>> GC_stopped_mark + 148
>>>>>>> 7   libgc.1.dylib                 0x000000010356d645
>>>>>>> GC_try_to_collect_inner + 245
>>>>>>> 8   libgc.1.dylib                 0x000000010356e302
>>>>>>> GC_collect_or_expand + 147
>>>>>>> 9   libgc.1.dylib                 0x000000010356e51a GC_allocobj +
>>>>>>> 228
>>>>>>> 10  libgc.1.dylib                 0x0000000103572515
>>>>>>> GC_generic_malloc_inner + 249
>>>>>>> 11  libgc.1.dylib                 0x00000001035725dd
>>>>>>> GC_generic_malloc + 79
>>>>>>> 12  libgc.1.dylib                 0x00000001035728c0 GC_core_malloc
>>>>>>> + 196
>>>>>>>
>>>>>>> --
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>>>>>>
>>>>>>
>>>>>
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>>>>
>>>>
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>>
>>
>
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