rb_sysref_compare
static int rb_sysref_compare(struct sysref *sr1, struct sysref *sr2);
RB_GENERATE2(sysref_rb_tree, sysref, rbnode, rb_sysref_compare, sysid_t, sysid);
RB_PROTOTYPE2(sysref_rb_tree, sysref, rbnode, rb_sysref_compare, sysid_t);