Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and cgroup_kill_sb()

[Tejun Heo]

Hello, Li.

Sorry about the long delay.

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

Thanks.

-- 
tejun
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