[2/3] cgroup: Iterate tasks that did not finish do_exit()
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Message ID 20200117151533.12381-3-mkoutny@suse.com
State New
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Series
  • cgroup: Iterate tasks that did not finish do_exit()
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Commit Message

Michal Koutný Jan. 17, 2020, 3:15 p.m. UTC
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>
---
 kernel/cgroup/cgroup.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Comments

Tejun Heo Jan. 17, 2020, 5:28 p.m. UTC | #1
On Fri, Jan 17, 2020 at 04:15:32PM +0100, Michal Koutný 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.

Yeah, this looks fine to me.  Any chance you can order this before the
clean up so that we can mark it for -stable.

Thanks.
Suren Baghdasaryan Jan. 17, 2020, 6:41 p.m. UTC | #2
On Fri, Jan 17, 2020 at 9:28 AM Tejun Heo <tj@kernel.org> wrote:
>
> On Fri, Jan 17, 2020 at 04:15:32PM +0100, Michal Koutný 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.

Tested-by: Suren Baghdasaryan <surenb@google.com>

>
> Yeah, this looks fine to me.  Any chance you can order this before the
> clean up so that we can mark it for -stable.
>

+1 for reordering. Makes it easier to backport.
Thanks,
Suren.

> Thanks.
>
> --
> tejun
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
>
Michal Koutný Jan. 20, 2020, 2:56 p.m. UTC | #3
On Fri, Jan 17, 2020 at 10:41:29AM -0800, Suren Baghdasaryan <surenb@google.com> wrote:
> Tested-by: Suren Baghdasaryan <surenb@google.com>
Thanks.

> > Yeah, this looks fine to me.  Any chance you can order this before the
> > clean up so that we can mark it for -stable.
> +1 for reordering. Makes it easier to backport.
The grounds still need to be prepared for css_task_iter to store
additional information. Let me see how the preceding changes can be
minimized.

Michal
Michal Koutný Jan. 24, 2020, 11:40 a.m. UTC | #4
Hi,
thanks for your feedback. I'm sending updated solution that changes task
iteration to be consistent with cgroup_is_populated() check and moving the
responsibility to check PF_EXITING on the consumers of iterator API.

Changes from v1
- v1: https://lore.kernel.org/lkml/20200117151533.12381-1-mkoutny@suse.com/
- Swap the fixing and refactoring patch (in order to make the fix easier for
  backport)
- Don't introduce tasks lists array because of unnecessarily long access code
- Fix comment in selftest
- I leave the CC:stable on discretion of the maintainer

Michal Koutný (2):
  cgroup: Iterate tasks that did not finish do_exit()
  cgroup: Clean up css_set task traversal

Suren Baghdasaryan (1):
  kselftest/cgroup: add cgroup destruction test

 include/linux/cgroup.h                     |   4 +-
 kernel/cgroup/cgroup.c                     |  60 ++++++-----
 tools/testing/selftests/cgroup/test_core.c | 113 +++++++++++++++++++++
 3 files changed, 146 insertions(+), 31 deletions(-)

Patch
diff mbox series

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index b56283e13491..132d258e7172 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4492,11 +4492,12 @@  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_list == CSS_SET_TASKS_DYING &&
+		    !atomic_read(&task->signal->live))
 			goto repeat;
 	} else {
 		/* skip all dying ones */
-		if (task->flags & PF_EXITING)
+		if (it->cur_list == CSS_SET_TASKS_DYING)
 			goto repeat;
 	}
 }