Re: [PATCH v2 1/3] cgroup: Iterate tasks that did not finish do_exit()

[Suren Baghdasaryan]

On Fri, Jan 24, 2020 at 3:40 AM Michal Koutnà <mkoutny@xxxxxxxx> wrote:
>
> PF_EXITING is set earlier than actual removal from css_set when a task
> is exitting. This can confuse cgroup.procs readers who see no PF_EXITING
> tasks, however, rmdir is checking against css_set membership so it can
> transitionally fail with EBUSY.
>
> Fix this by listing tasks that weren't unlinked from css_set active
> lists.
> It may happen that other users of the task iterator (without
> CSS_TASK_ITER_PROCS) spot a PF_EXITING task before cgroup_exit(). This
> is equal to the state before commit c03cd7738a83 ("cgroup: Include dying
> leaders with live threads in PROCS iterations") but it may be reviewed
> later.
>
> Reported-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Fixes: c03cd7738a83 ("cgroup: Include dying leaders with live threads in PROCS iterations")
> Signed-off-by: Michal Koutnà <mkoutny@xxxxxxxx>
> ---
>  include/linux/cgroup.h |  1 +
>  kernel/cgroup/cgroup.c | 23 ++++++++++++++++-------
>  2 files changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index d7ddebd0cdec..e75d2191226b 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -62,6 +62,7 @@ struct css_task_iter {
>         struct list_head                *mg_tasks_head;
>         struct list_head                *dying_tasks_head;
>
> +       struct list_head                *cur_tasks_head;
>         struct css_set                  *cur_cset;
>         struct css_set                  *cur_dcset;
>         struct task_struct              *cur_task;
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 735af8f15f95..a6e3619e013b 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -4404,12 +4404,16 @@ static void css_task_iter_advance_css_set(struct css_task_iter *it)
>                 }
>         } while (!css_set_populated(cset) && list_empty(&cset->dying_tasks));
>
> -       if (!list_empty(&cset->tasks))
> +       if (!list_empty(&cset->tasks)) {
>                 it->task_pos = cset->tasks.next;
> -       else if (!list_empty(&cset->mg_tasks))
> +               it->cur_tasks_head = &cset->tasks;
> +       } else if (!list_empty(&cset->mg_tasks)) {
>                 it->task_pos = cset->mg_tasks.next;
> -       else
> +               it->cur_tasks_head = &cset->mg_tasks;
> +       } else {
>                 it->task_pos = cset->dying_tasks.next;
> +               it->cur_tasks_head = &cset->dying_tasks;
> +       }
>
>         it->tasks_head = &cset->tasks;
>         it->mg_tasks_head = &cset->mg_tasks;
> @@ -4467,10 +4471,14 @@ static void css_task_iter_advance(struct css_task_iter *it)
>                 else
>                         it->task_pos = it->task_pos->next;
>
> -               if (it->task_pos == it->tasks_head)
> +               if (it->task_pos == it->tasks_head) {
>                         it->task_pos = it->mg_tasks_head->next;
> -               if (it->task_pos == it->mg_tasks_head)
> +                       it->cur_tasks_head = it->mg_tasks_head;
> +               }
> +               if (it->task_pos == it->mg_tasks_head) {
>                         it->task_pos = it->dying_tasks_head->next;
> +                       it->cur_tasks_head = it->dying_tasks_head;
> +               }
>                 if (it->task_pos == it->dying_tasks_head)
>                         css_task_iter_advance_css_set(it);
>         } else {
> @@ -4489,11 +4497,12 @@ static void css_task_iter_advance(struct css_task_iter *it)
>                         goto repeat;
>
>                 /* and dying leaders w/o live member threads */
> -               if (!atomic_read(&task->signal->live))
> +               if (it->cur_tasks_head == it->dying_tasks_head &&
> +                   !atomic_read(&task->signal->live))
>                         goto repeat;
>         } else {
>                 /* skip all dying ones */
> -               if (task->flags & PF_EXITING)
> +               if (it->cur_tasks_head == it->dying_tasks_head)
>                         goto repeat;
>         }
>  }
> --
> 2.24.1
>

Tested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Thanks!