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

Suren Baghdasaryan
2020-01-24T22:56:51Z

On Fri, Jan 24, 2020 at 3:40 AM Michal Koutný <mkoutny@suse.com> 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@google.com>
> Fixes: c03cd7738a83 ("cgroup: Include dying leaders with live threads in PROCS iterations")
> Signed-off-by: Michal Koutný <mkoutny@suse.com>
> ---
> 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@google.com>

Thanks!