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Subject[32/75] cgroups: fix a css_set not found bug in cgroup_attach_proc
3.1-stable review patch.  If anyone has any objections, please let me know.


From: Mandeep Singh Baines <>

commit e0197aae59e55c06db172bfbe1a1cdb8c0e1cab3 upstream.

There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
is not called for the PF_EXITING case, find_existing_css_set() will return
NULL inside cgroup_task_migrate() causing a BUG.

This bug is easy to reproduce. Create a zombie and echo its pid to

$ cat zombie.c
\#include <unistd.h>

int main()
if (fork())
return 0;

We are hitting this bug pretty regularly on ChromeOS.

This bug is already fixed by Tejun Heo's cgroup patchset which is
targetted for the next merge window:

I've create a smaller patch here which just fixes this bug so that a
fix can be merged into the current release and stable.

Signed-off-by: Mandeep Singh Baines <>
Reviewed-by: Li Zefan <>
Signed-off-by: Tejun Heo <>
Cc: KAMEZAWA Hiroyuki <>
Cc: Frederic Weisbecker <>
Cc: Oleg Nesterov <>
Cc: Andrew Morton <>
Cc: Paul Menage <>
Cc: Olof Johansson <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/cgroup.c | 5 -----
1 file changed, 5 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2098,11 +2098,6 @@ int cgroup_attach_proc(struct cgroup *cg
/* get old css_set pointer */
- if (tsk->flags & PF_EXITING) {
- /* ignore this task if it's going away */
- task_unlock(tsk);
- continue;
- }
oldcg = tsk->cgroups;

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