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Subject[PATCH cgroup/for-3.11-fixes] cgroup: fix rmdir EBUSY regression in 3.11
Hello, Hugh.

Oops, sorry about that. I massaged the patch a bit and applied it to
cgroup/for-3.11-fixes. Will send pull request to Linus right away.

Thanks.
------ 8< ------
From bb78a92f47696b2da49f2692b6a9fa56d07c444a Mon Sep 17 00:00:00 2001
From: Hugh Dickins <hughd@google.com>
Date: Wed, 28 Aug 2013 16:31:23 -0700

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>
---
kernel/cgroup.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 781845a..e919633 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -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);
@@ -4490,12 +4491,28 @@ 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
* css reference will be given out via css_tryget(). We can't
--
1.8.3.1


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