Messages in this thread | | | Date | Sun, 27 Aug 2023 18:18:38 -0700 | Subject | Re: [PATCH 3/6] bpf: task_group_seq_get_next: fix the skip_if_dup_files check | From | Yonghong Song <> |
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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. >
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