In ideal world, xorg should be capable of taking it by force.
I think this functionality has been broken for ages though.

I have disabled hw locking when entering span functions for now.

This relates to bug #6085.

foobillard segfault:
0xb77d00c8 in lerp_rgba_2d (result=0x0, a=0.5, b=0.5, t00=0xbfe7315c "", 
    t10=0xbfe73158 "", t01=0xbfe73154 "", t11=0xbfe73150 "")
    at swrast/s_texfilter.c:172
172        result[0] = ilerp_2d(ia, ib, t00[0], t10[0], t01[0], t11[0]);