[PATCH 19/98] cgroup: fix umount vs cgroup_cfts_commit() race

From: Luis Henriques
Date: Thu Jul 11 2013 - 10:50:14 EST


3.5.7.17 -stable review patch. If anyone has any objections, please let me know.

------------------

From: Li Zefan <lizefan@xxxxxxxxxx>

commit 084457f284abf6789d90509ee11dae383842b23b upstream.

cgroup_cfts_commit() uses dget() to keep cgroup alive after cgroup_mutex
is dropped, but dget() won't prevent cgroupfs from being umounted. When
the race happens, vfs will see some dentries with non-zero refcnt while
umount is in process.

Keep running this:
mount -t cgroup -o blkio xxx /cgroup
umount /cgroup

And this:
modprobe cfq-iosched
rmmod cfs-iosched

After a while, the BUG() in shrink_dcache_for_umount_subtree() may
be triggered:

BUG: Dentry xxx{i=0,n=blkio.yyy} still in use (1) [umount of cgroup cgroup]

Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
[ luis: backported to 3.5:
- fixed build error: added '&' to atomic_inc_not_zero parameter;
this was fixed upstream by commit e8c82d20a9f729cf4b9f73043f7fd4e0872bebfd ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
kernel/cgroup.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 07ba52a..3b3fa5b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2771,13 +2771,17 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
{
LIST_HEAD(pending);
struct cgroup *cgrp, *n;
+ struct super_block *sb = ss->root->sb;

/* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
- if (cfts && ss->root != &rootnode) {
+ if (cfts && ss->root != &rootnode &&
+ atomic_inc_not_zero(&sb->s_active)) {
list_for_each_entry(cgrp, &ss->root->allcg_list, allcg_node) {
dget(cgrp->dentry);
list_add_tail(&cgrp->cft_q_node, &pending);
}
+ } else {
+ sb = NULL;
}

mutex_unlock(&cgroup_mutex);
@@ -2800,6 +2804,9 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
dput(cgrp->dentry);
}

+ if (sb)
+ deactivate_super(sb);
+
mutex_unlock(&cgroup_cft_mutex);
}

--
1.8.1.2

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/