On Fri, Jan 24, 2020 at 2:56 PM Suren Baghdasaryan [off-list ref] wrote:

On Fri, Jan 24, 2020 at 3:40 AM Michal Koutný [off-list ref] wrote:

quoted

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!

Hi Folks,
If this new version looks good could we get an Ack please? I need to
start backporting this fix to Android and would like a confirmation
before doing so.
Thanks!

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