[Pixman] testsuite fails on power7

[Siarhei Siamashka]

On Tue, 17 Sep 2013 10:03:11 -0400
"Lennart Sorensen" <lsorense at csclub.uwaterloo.ca> wrote:

> On Mon, Sep 16, 2013 at 10:24:10PM +0200, Søren Sandmann wrote:
> > Lennart Sorensen <lsorense at csclub.uwaterloo.ca> writes:
> > 
> > > And to make it even more annoying to track down:
> > >
> > > It doesn't fail on a power6, only on a power7.  power7 machines are
> > > known to have found numerous powerpc memory barrier bugs in code
> > > (including compiler and library code), where earlier generations let
> > > you get away with stuff that the architecture didn't actually allow,
> > > but which usually worked.
> > >
> > > So it seems to be a bug trigged by: vmx code used with openmp with libc
> > > 2.13 on power7.  Change any one of those 4 things, and the bug
> > > vanishes.
> > 
> > As far as I can see, this is all still consistent with the bug being
> > that the VMX combiners are writing outside the malloced memory:
> > 
> > - Disable OpenMP, and it doesn't matter because the same bytes are read
> >   and written. With OpenMP, two threads can mess each other's memory up.
> > 
> > - Use different libc: malloc() may allocate different amounts of memory
> >   so that the combiners don't write outside of the allocated area.
> > 
> > - Disable VMX: There is no writing outside the malloc()ed area
> > 
> > - Power 6: Could just be timing differences, but may also have to do
> >   with different atomicity of the incorrect memory accesses.
> 
> The thing is that if I set the openmp environment to only use one thread,
> the problem does NOT go away.  So either I did it wrong, or it's a bit
> more tricky than that.
> 
> But perhaps it really is just that the unaligned vector operations are
> smashing each other.
> 
> I did see one altivec document showing the use of vec_ste rather than
> vec_st in a complex manner to do unaligned stores with a comment about
> thread safety.  It may be that vec_st is NOT thread safe for unaligned
> stores.
> 
> I did try increasing the malloc size by some extra margin amount, and
> that did not help at all.  If it was just each thread going into the
> range of the bytes used by another thread, I would not expect that to
> corrupt libc's data structures for the malloc, which certainly seems to
> be happening.  libc complains about double free or corrupt linked lists
> on some runs (other times it just segfaults).

Could you please give the following patches a try in your problematic
configuration on power7?

    http://lists.freedesktop.org/archives/pixman/2013-September/002969.html

They are also available here:

    https://github.com/ssvb/pixman/commits/20130927-vmx-fix-unaligned-writes

-- 
Best regards,
Siarhei Siamashka