Hello, Kamezawa-san.

On Sat, Jul 21, 2012 at 11:14:21AM +0900, Kamezawa Hiroyuki wrote:

I'm sorry I misunderstand. The problem is following.

        CPU A                       CPU B
    mutex_unlock()
                                mutex_lock()
    ->pre_destroy()             attach task
      commit res->usage=0       mutex_unlock()
                                increase res->usage
                                detach task
    mutex_lock()
    check css's refcount=0
     ....continue destroy.

Now, I thinks memcg's check is not enough but putting the -EBUSY there
not to forget this race.


I think a patch to stop task-attach and create child cgroup if  CGRP_WAIT_ON_RMDIR
is set is required. And that's enough..

The *ONLY* reason we're not marking the cgroup dead after the checking
whether the cgroup has children or task at the top of cgroup_rmdir()
is because memcg might fail ->pre_destroy() and cancel the cgroup
removal.  We can't commit to removal because memcg might fail.

Now, if memcg drops the deprecated behavior, we can simply commit to
removal *before* starting calling pre_destroy() and it doesn't matter
at all whether we hold cgroup_mutex across pre_destroy or not and
cgroup core will simply deny any addition to the cgroup committed to
death.  (and remove a handsome amount of ugly code in the process)

So, the *ONLY* reason this can't be fixed properly from cgroup core is
because memcg's pre_destory() might fail and it doesn't make much
sense to me to implement add a workaround at this point when the whole
problem will go away once memcg's pre_destroy() is updated.

So, please update memcg and drop the __DEPRECATED flag, so that the
cgroup core can drop at least this particular part of misdesign. :(

Thanks.

-- 
tejun

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