Re: [PATCH 1/3] bpf: task_group_seq_get_next: use __next_thread() rather than next_thread()

[Yonghong Song <yonghong.song@linux.dev>]

On 11/16/23 4:34 AM, Oleg Nesterov wrote:
> On 11/15, Yonghong Song wrote:
>> On 11/14/23 11:32 AM, Oleg Nesterov wrote:
>>> @@ -70,15 +70,13 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>>>   		return NULL;
>>>   retry:
>>> -	task = next_thread(task);
>>> +	task = __next_thread(task);
>>> +	if (!task)
>>> +		return NULL;
>>>   	next_tid = __task_pid_nr_ns(task, PIDTYPE_PID, common->ns);
>>> -	if (!next_tid || next_tid == common->pid) {
>>> -		/* Run out of tasks of a process.  The tasks of a
>>> -		 * thread_group are linked as circular linked list.
>>> -		 */
>>> -		return NULL;
>>> -	}
>>> +	if (!next_tid)
>>> +		goto retry;
>> Look at the code. Looks like next_tid should never be 0
> ...
>
>> pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
>>                          struct pid_namespace *ns)
>> {
>>          pid_t nr = 0;
>>
>>          rcu_read_lock();
>>          if (!ns)
>>                  ns = task_active_pid_ns(current);
>>          nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
>                                            ^^^^^^^^^^^^^^^^^^^^^^^^^
>
> Please note that task_pid_ptr(task, type)) can return NULL if this
> task has already exited and called detach_pid().
>
> detach_pid() does __change_pid(task, type, NULL), please note the
>
> 	*pid_ptr = new; // NULL in this case
>
> assignment in __change_pid().
>
> IOW. The problem is not that ns can change, the problem is that
> task->thread_pid (and other pid links) can be NULL, and in this
> case pid_nr_ns() returns zero.

Thanks for explanation. I certainly missed race between task
iterator and __change_pid(). Then the patch looks good to me.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

>
>
> This code should be rewritten from the very beginning, it should
> not rely on pid_nr. If nothing else common->pid and/or pid_visiting
> can be reused. But currently my only concern is next_thread().
>
>> Other than above, the change looks good to me.
> Thanks for review!
>
> Oleg.
>