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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