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

Michal Koutný
2020-01-24T11:40:15Z

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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Fixes: c03cd7738a83 ("cgroup: Include dying leaders with live threads in PROCS iterations")
Signed-off-by: Michal Koutn√Ω <mkoutny-IBi9RG/b67k@public.gmane.org>
---
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