Commit 3a32bd72 authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
Browse files

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: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 4e26445f

kernel/cgroup.c

+26 −7
Original line number Diff line number Diff line
@@ -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.

		 */

		if (!percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {

		 * 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.

		 */

		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;

}

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