Argh, I wish I could have found this report earlier! I discovered it by doing a
Google search on commit edbbfac6cfc634e697d7f981155a5072c52d77ac which
introduced the exact same lockup for my video card. I've been maintaining the
following report for years now, about the same bug apparently:

https://bugs.freedesktop.org/show_bug.cgi?id=73528

So from what I understand, setting R600_DEBUG=notiling should prevent
fast-clear from freezing the GPU? I will gladly try this for Second Life and
other programs that trigger the problem, and see if there are any changes.