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Subject[PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy (v2)
This is fixed one. Thank you for all help.

Regards,
-Kame
==
As Balbir pointed out, memcg's pre_destroy handler has potential deadlock.

It has following lock sequence.

cgroup_mutex (cgroup_rmdir)
-> pre_destroy -> mem_cgroup_pre_destroy-> force_empty
-> cpu_hotplug.lock. (lru_add_drain_all->
schedule_work->
get_online_cpus)

But, cpuset has following.
cpu_hotplug.lock (call notifier)
-> cgroup_mutex. (within notifier)

Then, this lock sequence should be fixed.

Considering how pre_destroy works, it's not necessary to holding
cgroup_mutex() while calling it.

As side effect, we don't have to wait at this mutex while memcg's force_empty
works.(it can be long when there are tons of pages.)

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

---
kernel/cgroup.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

Index: mmotm-2.6.28-Nov10/kernel/cgroup.c
===================================================================
--- mmotm-2.6.28-Nov10.orig/kernel/cgroup.c
+++ mmotm-2.6.28-Nov10/kernel/cgroup.c
@@ -2475,10 +2475,7 @@ static int cgroup_rmdir(struct inode *un
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
-
- parent = cgrp->parent;
- root = cgrp->root;
- sb = root->sb;
+ mutex_unlock(&cgroup_mutex);

/*
* Call pre_destroy handlers of subsys. Notify subsystems
@@ -2486,7 +2483,14 @@ static int cgroup_rmdir(struct inode *un
*/
cgroup_call_pre_destroy(cgrp);

- if (cgroup_has_css_refs(cgrp)) {
+ mutex_lock(&cgroup_mutex);
+ parent = cgrp->parent;
+ root = cgrp->root;
+ sb = root->sb;
+
+ if (atomic_read(&cgrp->count)
+ || !list_empty(&cgrp->children)
+ || cgroup_has_css_refs(cgrp)) {
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}


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