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cgroup: fix rmdir EBUSY regression in 3.11

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On 3.11-rc we are seeing cgroup directories left behind when they should
have been removed.  Here's a trivial reproducer:

cd /sys/fs/cgroup/memory
mkdir parent parent/child; rmdir parent/child parent
rmdir: failed to remove `parent': Device or resource busy

It's because cgroup_destroy_locked() (step 1 of destruction) leaves
cgroup on parent's children list, letting cgroup_offline_fn() (step 2 of
destruction) remove it; but step 2 is run by work queue, which may not
yet have removed the children when parent destruction checks the list.

Fix that by checking through a non-empty list of children: if every one
of them has already been marked CGRP_DEAD, then it's safe to proceed:
those children are invisible to userspace, and should not obstruct rmdir.

(I didn't see any reason to keep the cgrp->children checks under the
unrelated css_set_lock, so moved them out.)

tj: Flattened nested ifs a bit and updated comment so that it's
    correct on both for-3.11-fixes and for-3.12.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Hugh Dickins 2013-08-28 16:31:23 -07:00 committed by Tejun Heo
parent c95389b4cd
commit bb78a92f47
1 changed files with 18 additions and 1 deletions
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19
kernel/cgroup.c
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@ -4480,6 +4480,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
struct dentry *d = cgrp->dentry;
struct cgroup_event *event, *tmp;
struct cgroup_subsys *ss;
struct cgroup *child;
bool empty;
lockdep_assert_held(&d->d_inode->i_mutex);
@ -4490,11 +4491,27 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
* @cgrp from being removed while __put_css_set() is in progress.
*/
read_lock(&css_set_lock);
empty = list_empty(&cgrp->cset_links) && list_empty(&cgrp->children);
empty = list_empty(&cgrp->cset_links);
read_unlock(&css_set_lock);
if (!empty)
return -EBUSY;
/*
* Make sure there's no live children. We can't test ->children
* emptiness as dead children linger on it while being destroyed;
* otherwise, "rmdir parent/child parent" may fail with -EBUSY.
*/
empty = true;
rcu_read_lock();
list_for_each_entry_rcu(child, &cgrp->children, sibling) {
empty = cgroup_is_dead(child);
if (!empty)
break;
}
rcu_read_unlock();
if (!empty)
return -EBUSY;
/*
* Block new css_tryget() by killing css refcnts. cgroup core
* guarantees that, by the time ->css_offline() is invoked, no new