I enabled CONFIG_PROVE_LOCKING=y and added slub_debug=FZPU in order to track down https://bugs.freedesktop.org/show_bug.cgi?id=57122 . While running the recompiled kernel with CONFIG_ZCACHE2=y, I got a "INFO: possible irq lock inversion dependency 
detected" message in the kernel log. I was told to report it here.