[PATCH] fix do_coredump() vs SIGSTOP race

Let's suppose we have 2 threads in thread group:
A - does coredump
B - has pending SIGSTOP

thread A thread B

do_coredump: get_signal_to_deliver:

lock(->sighand)
->signal->flags = SIGNAL_GROUP_EXIT
unlock(->sighand)

lock(->sighand)
signr = dequeue_signal()
->signal->flags |= SIGNAL_STOP_DEQUEUED
return SIGSTOP;

do_signal_stop:
unlock(->sighand)

coredump_wait:

zap_threads:
lock(tasklist_lock)
send SIGKILL to B
// signal_wake_up() does nothing
unlock(tasklist_lock)

lock(tasklist_lock)
lock(->sighand)
re-check sig->flags & SIGNAL_STOP_DEQUEUED, yes
set_current_state(TASK_STOPPED);
finish_stop:
schedule();
// ->state == TASK_STOPPED

wait_for_completion(&startup_done)
// waits for complete() from B,
// ->state == TASK_UNINTERRUPTIBLE

We can't wake up 'B' in any way:

SIGCONT will be ignored because handle_stop_signal() sees
->signal->flags & SIGNAL_GROUP_EXIT.

sys_kill(SIGKILL)->__group_complete_signal() will choose
uninterruptible 'A', so it can't help.

sys_tkill(B, SIGKILL) will be ignored by specific_send_sig_info()
because B already has pending SIGKILL.

This scenario is not possbile if 'A' does do_group_exit(), because
it sets sig->flags = SIGNAL_GROUP_EXIT and delivers SIGKILL to
subthreads atomically, holding both tasklist_lock and sighand->lock.
That means that do_signal_stop() will notice !SIGNAL_STOP_DEQUEUED
after re-locking ->sighand. And it is not possible to any other
thread to re-add SIGNAL_STOP_DEQUEUED later, because dequeue_signal()
can only return SIGKILL.

I think it is better to change do_coredump() to do sigaddset(SIGKILL)
and signal_wake_up() under sighand->lock, but this patch is much
simpler.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>