Re: [PATCH 3/6] bpf: task_group_seq_get_next: fix the skip_if_dup_files check

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

On 8/27/23 1:19 PM, Oleg Nesterov wrote:
> On 08/25, Yonghong Song wrote:
>>
>> On 8/25/23 10:04 AM, Oleg Nesterov wrote:
>>> Forgot to mention in the changelog...
>>>
>>> In any case this doesn't look right. ->group_leader can exit before other
>>> threads, call exit_files(), and in this case task_group_seq_get_next() will
>>> check task->files == NULL.
>>
>> It is okay. This won't be affecting correctness. We will end with
>> calling bpf program for 'next_task'.
> 
> Well, I didn't mean it is necessarily wrong, I simply do not know.
> 
> But let's suppose that we have a thread group with the main thread M + 1000
> sub-threads. In the likely case they all have the same ->files, CLONE_THREAD
> without CLONE_FILES is not that common.
> 
> Let's assume the BPF_TASK_ITER_TGID case for simplicity.
> 
> Now lets look at task_file_seq_get_next() which passes skip_if_dup_files == 1
> to task_seq_get_next() and thus to task_group_seq_get_next().
> 
> Now, in this case task_seq_get_next() will return non-NULL only once (OK, unless
> task_file_seq_ops.stop() was called), it will return the group leader M first,
> then after task_file_seq_get_next() "reports" all the fd's of M and increments
> info->tid, the next task_seq_get_next(&info->tid, true) should return NULL because
> of the skip_if_dup_files check in task_group_seq_get_next().
> 
> Right?
> 
> But. if the group leader M exits then M->files == NULL. And in this case
> task_seq_get_next() will need to "inspect" all the sub-threads even if they all
> have the same ->files pointer.

That is correct. I do not have practical experience on how much
possibility this scenario may happen. I assume it should be very low.
If this is not the case, we might need to revisit.

> 
> No?
> 
> Again, I am not saying this is a bug and quite possibly I misread this code, but
> in any case the skip_if_dup_files logic looks sub-optimal and confusing to me.
> 
> Nevermind, please forget. This is minor even if I am right.
> 
> Thanks for rewiev!
> 
> Oleg.
>