Commit 
e56d090310d7625ecb43a1eeebd479f04affb48b
   [PATCH] RCU signal handling
made this BUG_ON() unsafe. This code runs under ->siglock,
while switch_exec_pids() takes tasklist_lock.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
                if (t == NULL)
                        /* restart balancing at this thread */
                        t = p->signal->curr_target = p;
-               BUG_ON(t->tgid != p->tgid);
 
                while (!wants_signal(sig, t)) {
                        t = next_thread(t);