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cgroup: fix umount vs cgroup_cfts_commit() race

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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@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
This commit is contained in:
Li Zefan 2013-06-18 18:40:19 +08:00 committed by Tejun Heo
parent 6db8e85c5c
commit 084457f284
1 changed files with 8 additions and 1 deletions
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9
kernel/cgroup.c
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@ -2798,13 +2798,17 @@ 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);
@ -2827,6 +2831,9 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
dput(cgrp->dentry);
}
if (sb)
deactivate_super(sb);
mutex_unlock(&cgroup_cft_mutex);
}