On 16/07/2010 14:04, Axel Simon wrote:
Hi Simon,
On 16.07.2010, at 14:29, Simon Marlow wrote:
On 16/07/2010 12:36, Axel Simon wrote:
Dear Haskell maintainers,
I've progressed a little and found that the problem is down to
accessing global variables that are declared in dynamic libraries. In
a nutshell, this doesn't as the addresses of these global variables
are all wrong when ghci is executing the code. So, I think I hit:
http://hackage.haskell.org/trac/ghc/ticket/781
I was able to work around this problem by compiling the C modules with
-fPIC. This bug is pretty bad, I'd say. I've added myself to its CC
list.
Urgh. It's a nasty bug, but not one that we can fix, because it's an
artifact of the small memory model used on x86_64. The only fix is to
use -fPIC.
It might be possible to use -fPIC either by default, or perhaps just
for .c files and when compiling data references from FFI declarations
in Haskell code, that's something we could look into. We might want
-fPIC on by default anyway if we switch to using dynamic linking by
default (but we're not yet sure what ramifications that will have).
Well, my fix is:
if arch(x86_64)
cc-options: -fPIC
This only affects the C files we compile of which there are only two at
the moment. I am happy with this solution since I know which files are
affected.
But basically this bug will hit me whenever I use a global C variable
from within Haskell? I hope there are none that we use, they should all
be accessed using functions, so we should be safe.
A reference to data that resides in a shared library, yes. It's
surprising how rarely this happens in fact.
Cheers,
Simon
Cheers,
Axel
Cheers,
Simon
Cheers,
Axel
On 14.07.2010, at 16:51, Axel Simon wrote:
Hi all,
I'm trying to debug a segfault relating to the memory management in
Gtk2Hs. Rather than make you read the ticket
http://hackage.haskell.org/trac/gtk2hs/ticket/1183
, I'll describe the problem:
- compiler 6.12.1 or 6.12.3
- darcs head of Gtk2Hs with #define DEBUG instead of #undef DEBUG in
gtk/Graphics/UI/Gtk/General/hsthread.c
- platform Ubuntu Linux, x86-64
- to reproduce: cd gtk2hs/gtk/demo/hello and run ghci World.hs and
type 'main'
A window with the "Hello World" button appears. After a few seconds,
the GC runs and the finaliser of the GtkButton is run since the
Haskell program no longer holds a reference to that object (only the
GtkWindow in C land has).
Thus, the GC calls a C function gtk2hs_g_object_unref_from_mainloop
which is supposed to enqueue the object into a global data structure
from which objects are later taken and g_object_unref is called on
them.
This global data structure is protected by a mutex, which is
acquired using g_static_mutex_lock:
void gtk2hs_g_object_unref_from_mainloop(gpointer object) {
int mutex_locked = 0;
if (threads_initialised) {
#ifdef DEBUG
printf("acquiring lock to add a %s object at %lx\n",
g_type_name(G_OBJECT_TYPE(object)), (unsigned long)
object);
printf("value of lock function is %lx\n",
(unsigned long)
g_thread_functions_for_glib_use.mutex_lock);
#endif
g_rand_new();
#if defined( WIN32 )
EnterCriticalSection(>k2hs_finalizer_mutex);
#else
g_static_mutex_lock(>k2hs_finalizer_mutex);
#endif
mutex_locked = 1;
}
[..]
The program prints:
acquiring lock to add a GtkButton object at 22d8020
value of lock function is 0
zsh: segmentation fault ghci World
Now the debugging weirdness starts. Whatever I do, I cannot get gdb
to find the symbol gtk2hs_g_object_unref_from_mainloop.
Since the function above is contained in a C file that comes with
our Haskell library, I tried to add "cc-options: -g" and "cc-
options: -ggdb -O0", but maybe somewhere symbols are stripped. So I
added the bogus function call to "g_rand_new()" which is not called
anywhere else and gdb stops as follows:
acquiring lock to add a GtkButton object at 2105020
value of lock function is 0
[Switching to Thread 0x7ffff41ff710 (LWP 15735)]
Breakpoint 12, 0x00007ffff115bfa0 in g_rand_new () from /usr/lib/
libglib-2.0.so
This all seems reasonable, but:
(gdb) bt
#0 0x00007ffff115bfa0 in g_rand_new () from /usr/lib/libglib-2.0.so
#1 0x00000000419b3792 in ?? ()
#2 0x00007ffff678f078 in ?? ()
i.e. the calling context is broken. I'm very, very sure that the
caller is indeed the above mentioned function and since g_rand_new
isn't called anywhere in my Haskell program (and otherwise the
calling context would be sane).
I'm also passing the address of gtk2hs_g_object_unref_from_mainloop
as FinalizerPtr to all my ForeignPtrs, so there is no inlining going
on.
Back to the culprit, the call to g_static_mutex_lock. This is a
macro that expands to
*g_thread_functions_for_glib_use.mutex_lock
where g_thread_functions_for_glib is a global variable that contains
a lot of function pointers. At the break point, it contains this:
(gdb) print g_thread_functions_for_glib_use
$33 = {mutex_new = 0x7ffff0cd9820<g_mutex_new_posix_impl>,
mutex_lock = 0x7ffff6c8b3c0<__pthread_mutex_lock>,
mutex_trylock = 0x7ffff0cd97b0<g_mutex_trylock_posix_impl>,
mutex_unlock = 0x7ffff6c8ca00<__pthread_mutex_unlock>,
mutex_free = 0x7ffff0cd9740<g_mutex_free_posix_impl>,
[..]
So the call to g_mutex_lock should call the function
__pthread_mutex_lock but it calls NULL.
I hoped that writing this email would give me a bit more insight
into the problem, but for now I suspect that something overwrites
either the stack or the code of the function.
On the same platform, the compiled version prints:
acquiring lock to add a GtkButton object at 1b05820
value of lock function is 7f7adcabd3c0
within mutex: adding finalizer to a GtkButton object!
On Mac OS or i386, using ghci or ghc, version 6.10.4, it works as
well.
Now for the fun bit: on i386 using ghci version 6.12.1 it works too.
So it's an x86-64 and ghc 6.12.1 bug. According to Christian Maeder
who submitted the ticket, the problem persists in 6.12.3.
Any hints and help appreciated,
Cheers,
Axel
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