On 2014/6/21 3:35, Tejun Heo wrote:

Hello, Li.

Sorry about the long delay.

On Thu, Jun 12, 2014 at 02:33:05PM +0800, Li Zefan wrote:

quoted

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 increase the refcnt of the superblock instead of increasing
the percpu refcnt of cgroup root.

Ah, right.  Gees, I'm really hating the fact that we have ->mount but
not ->umount.  However, can't we make it a bit simpler by just
introducing a mutex protecting looking up and refing up an existing
root and a sb going away?  The only problem is that the refcnt being
killed isn't atomic w.r.t. new live ref coming up, right?  Why not
just add a mutex around them so that they can't race?

Well, kill_sb() is called with sb->s_umount held, while kernfs_mount()
returned with sb->s_umount held, so adding a mutex will lead to ABBA
deadlock.