Commit 084457f2 authored Jun 18, 2013 by Li Zefan Committed by Tejun Heo Jun 18, 2013

cgroup: fix umount vs cgroup_cfts_commit() race



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

parent 6db8e85c

kernel/cgroup.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -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);
		}

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