Oleg Nesterov <oleg@xxxxxxxxxx> writes:
> On 03/03, Eric W. Biederman wrote:
>> @@ -1065,11 +1065,8 @@ static int de_thread(struct task_struct *tsk)
>> }
>>
>> sig->group_exit_task = tsk;
>> - sig->notify_count = zap_other_threads(tsk);
>> - if (!thread_group_leader(tsk))
>> - sig->notify_count--;
>> -
>> - while (sig->notify_count) {
>> + zap_other_threads(tsk);
>> + while (atomic_read(&sig->live) > 1) {
>> __set_current_state(TASK_KILLABLE);
>> spin_unlock_irq(lock);
>> schedule();
>
> Very nice. So de_thread() returns as soon as all other threads decrement
> signal->live in do_exit(). Before they do, say, exit_mm(). This is already
> wrong, for example this breaks OOM. Plus a lot more problems afaics, but
> lets ignore this.
Which means that we need to keep sig->notify_count.
> Note that de_thread() also unshares ->sighand before return. So in the
> case of mt exec it will likely see oldsighand->count != 1 and alloc the
> new sighand_struct and this breaks the locking.
>
> Because the execing thread will use newsighand->siglock to protect its
> signal_struct while the zombie threads will use oldsighand->siglock to
> protect the same signal struct. Yes, tasklist_lock + the fact irq_disable
> implies rcu_lock mostly save us but not entirely, say, a foreign process
> doing __send_signal() can take the right or the wrong lock depending on
> /dev/random.
Which leads to the question how can we get back tot he 2.4 behavior
of freeing sighand_struct in do_exit?
At which point as soon as we free sighand_struct if we are the last
to dying thread notify de_thread and everything works.
There are only two uses of sighand->siglock that I can see in
release_task: __ptrace_unlink and __exit_signal.
For what __ptrace_unlink is doing we should just be able to skip
acquiring of siglock if PF_EXITING is set.
__exit_signal is a little more interesting but half of what it is
doing looks like it was pulled out of do_exit and just needs to
be put back.
Which probably adds up to 4 or 5 small carefully written patches to sort
out that part of the exit path, but I think it leads to a very simple
and straight forward result. With the only side effect that we
can exec a process and still have a debugger reaping zombie threads.
Oleg does that sound reasonable to you?
Eric
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