cgroup: fix a race between cgroup_mount() and cgroup_kill_sb()

We've converted cgroup to kernfs so cgroup won't be intertwined with
vfs objects and locking, but there are dark areas.

Run two instances of this script concurrently:

    for ((; ;))
    {
    	mount -t cgroup -o cpuacct xxx /cgroup
    	umount /cgroup
    }

After a while, I saw two mount processes were stuck at retrying, because
they were waiting for a subsystem to become free, but the root associated
with this subsystem never got freed.

This can happen, if thread A is in the process of killing superblock but
hasn't called percpu_ref_kill(), and at this time thread B is mounting
the same cgroup root and finds the root in the root list and performs
percpu_ref_try_get().

To fix this, we try to increase both the refcnt of the superblock and the
percpu refcnt of cgroup root.

v2:
- we should try to get both the superblock refcnt and cgroup_root refcnt,
  because cgroup_root may have no superblock assosiated with it.
- adjust/add comments.

tj: Updated comments.  Renamed @sb to @pinned_sb.

Cc: <stable@vger.kernel.org> # 3.15
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 26 insertions, 7 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 64068667be84..70776aec2562 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1648,6 +1648,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                          int flags, const char *unused_dev_name,
                          void *data)
 {
+        struct super_block *pinned_sb = NULL;
         struct cgroup_subsys *ss;
         struct cgroup_root *root;
         struct cgroup_sb_opts opts;
@@ -1740,15 +1741,23 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                 }
 
                 /*
-                 * A root's lifetime is governed by its root cgroup.
-                 * tryget_live failure indicate that the root is being
-                 * destroyed.  Wait for destruction to complete so that the
-                 * subsystems are free.  We can use wait_queue for the wait
-                 * but this path is super cold.  Let's just sleep for a bit
-                 * and retry.
+                 * We want to reuse @root whose lifetime is governed by its
+                 * ->cgrp.  Let's check whether @root is alive and keep it
+                 * that way.  As cgroup_kill_sb() can happen anytime, we
+                 * want to block it by pinning the sb so that @root doesn't
+                 * get killed before mount is complete.
+                 *
+                 * With the sb pinned, tryget_live can reliably indicate
+                 * whether @root can be reused.  If it's being killed,
+                 * drain it.  We can use wait_queue for the wait but this
+                 * path is super cold.  Let's just sleep a bit and retry.
                  */
-                if (!percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
+                pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
+                if (IS_ERR(pinned_sb) ||
+                    !percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
                         mutex_unlock(&cgroup_mutex);
+                        if (!IS_ERR_OR_NULL(pinned_sb))
+                                deactivate_super(pinned_sb);
                         msleep(10);
                         ret = restart_syscall();
                         goto out_free;
@@ -1793,6 +1802,16 @@ out_free:
                                 CGROUP_SUPER_MAGIC, &new_sb);
         if (IS_ERR(dentry) || !new_sb)
                 cgroup_put(&root->cgrp);
+
+        /*
+         * If @pinned_sb, we're reusing an existing root and holding an
+         * extra ref on its sb.  Mount is complete.  Put the extra ref.
+         */
+        if (pinned_sb) {
+                WARN_ON(new_sb);
+                deactivate_super(pinned_sb);
+        }
+
         return dentry;
 }