After some days of recompiling with debug symbols and stepping endlessly through programs, I have become fairly sure that this problem has nothing at all to do with DirectFb. Sorry for the noise. It just happened that I was inside DirectFB mostly when the crash occurred, but after recompiling SDL the default driver changed from DirectFB to X11 and still the crashes happened, in fairly random locations. SDL Init seems to trigger some process or thread that eventually causes the complete crash of X sometime thereafter.
As an experiment I set my video device to fbdev in xorg.conf and suddenly everything started working, though obviously slowly. So it seems the problem must be in xserver-xorg-video-radeon.
