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authorTejun Heo <tj@kernel.org>2012-07-07 16:08:18 -0700
committerTejun Heo <tj@kernel.org>2012-07-07 16:08:18 -0700
commit5db9a4d99b0157a513944e9a44d29c9cec2e91dc (patch)
tree8373f346ea412ece84781bdce33c5e938ad3e3ee
parent7db5b3ca0ecdb2e8fad52a4770e4e320e61c77a6 (diff)
cgroup: fix cgroup hierarchy umount race
48ddbe1946 "cgroup: make css->refcnt clearing on cgroup removal optional" allowed a css to linger after the associated cgroup is removed. As a css holds a reference on the cgroup's dentry, it means that cgroup dentries may linger for a while. Destroying a superblock which has dentries with positive refcnts is a critical bug and triggers BUG() in vfs code. As each cgroup dentry holds an s_active reference, any lingering cgroup has both its dentry and the superblock pinned and thus preventing premature release of superblock. Unfortunately, after 48ddbe1946, there's a small window while releasing a cgroup which is directly under the root of the hierarchy. When a cgroup directory is released, vfs layer first deletes the corresponding dentry and then invokes dput() on the parent, which may recurse further, so when a cgroup directly below root cgroup is released, the cgroup is first destroyed - which releases the s_active it was holding - and then the dentry for the root cgroup is dput(). This creates a window where the root dentry's refcnt isn't zero but superblock's s_active is. If umount happens before or during this window, vfs will see the root dentry with non-zero refcnt and trigger BUG(). Before 48ddbe1946, this problem didn't exist because the last dentry reference was guaranteed to be put synchronously from rmdir(2) invocation which holds s_active around the whole process. Fix it by holding an extra superblock->s_active reference across dput() from css release, which is the dput() path added by 48ddbe1946 and the only one which doesn't hold an extra s_active ref across the final cgroup dput(). Signed-off-by: Tejun Heo <tj@kernel.org> LKML-Reference: <4FEEA5CB.8070809@huawei.com> Reported-by: shyju pv <shyju.pv@huawei.com> Tested-by: shyju pv <shyju.pv@huawei.com> Cc: Sasha Levin <levinsasha928@gmail.com> Acked-by: Li Zefan <lizefan@huawei.com>
-rw-r--r--kernel/cgroup.c6
1 files changed, 5 insertions, 1 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5f134a0e0e3..b303dfc7dce 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3883,8 +3883,12 @@ static void css_dput_fn(struct work_struct *work)
{
struct cgroup_subsys_state *css =
container_of(work, struct cgroup_subsys_state, dput_work);
+ struct dentry *dentry = css->cgroup->dentry;
+ struct super_block *sb = dentry->d_sb;
- dput(css->cgroup->dentry);
+ atomic_inc(&sb->s_active);
+ dput(dentry);
+ deactivate_super(sb);
}
static void init_cgroup_css(struct cgroup_subsys_state *css,