[Qt-interest] SegFaults I cannot interpret

[Andrea Franceschini]

Hello guys. Since I started developing this application I'm working
on, I've had any sort of segfault, sometimes due to errors of mine --
which have been fixed -- and sometimes due to conditions I cannot
really grasp. I fear the problem may be somehow related to threading
issues but it only appears sometimes while other times I can run
flawlessly for a lot of time.

Basically I have a QPainter derived class which has pointers to my
datas and displays them on an appropriate QGLWidget. I tried to put
mutexes everywhere (nontheless I tried with the QMutexLocker if by any
chance I was missing something) but I found that it really affected
the performance of the painter (while the logic still worked really
well).

Most of the sgefaults I have look like

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb6d8c710 (LWP 31154)]
0x0804c96a in QBasicAtomicInt::ref (this=0x0) at
/usr/include/qt4/QtCore/qatomic_i386.h:115
115                      : "memory");
(gdb) bt
#0  0x0804c96a in QBasicAtomicInt::ref (this=0x0) at
/usr/include/qt4/QtCore/qatomic_i386.h:115
#1  0x0804d133 in QList (this=0xbffbd520, l=@0x866fec8) at
/usr/include/qt4/QtCore/qlist.h:106
#2  0x080513af in Doodle::Painter::paint (this=0x81cff10,
painter=0xbffbd63c, event=0xbffbdaf4) at Painter.cpp:95
...

or

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb6e86710 (LWP 30961)]
0x0804c99e in QListData::size (this=0xa2b54a8) at
/usr/include/qt4/QtCore/qlist.h:82
82          inline int size() const { return d->end - d->begin; }
(gdb) bt
#0  0x0804c99e in QListData::size (this=0xa2b54a8) at
/usr/include/qt4/QtCore/qlist.h:82
#1  0x0804ee01 in QList<QList<Doodle::Cursor> >::size (this=0xa2b54a8)
at /usr/include/qt4/QtCore/qlist.h:112
#2  0x08051219 in Doodle::Painter::paint (this=0x9e80f10,
painter=0xbf8b972c, event=0xbf8b9be4) at Painter.cpp:96
...

(despite of the line number, the instruction was the same and I
double-makeittriple-checked: it shouldn't return null pointers or any
other kind of bad stuff).

Clearly these are something that go far beyond my ability of
inspection (and patience, since I'm working on strict time constraint
and a single step-debug session would take an entire day and even
wouldn't give any warranty of producing the crash at all) so if
anybody of you guys can give me some insight, I'm totally lost.

Thanks
-- 
Andrea