cgroup attach/fork hooks consistency with the ns_cgroup

[Daniel Lezcano]

Hi,

I noticed two different behaviours, the second one looks weird for me:

  1) when the cgroup is manually created:
	mkdir /cgroup/foo
	echo $$ > /cgroup/foo/tasks

  only the "attach" callback is called as expected.

  2) when the cgroup is automatically created via the ns_cgroup with the 
clone function and the namespace flags,

   the "attach" *and* the "fork" callbacks are called.


IMHO, these two different behaviours look inconsistent. Won't this lead 
to some problems or a specific code to handle both cases if a cgroup is 
using the fork and the attach hooks ?

For example, let's imagine we create a control group which shows the 
number of tasks running. We have a global atomic and we display its 
value in the cgroupfs.

When a task attaches to the cgroup, we do atomic_inc in the attach 
callback. For all its child, the fork hook will do atomic_inc and exit 
hook will do atomic_dec.

If we create the cgroup manually like the case 1) that works. But if we 
use the control group with the ns_cgroup the task counter will be set to 
2 for the first tasks entering the cgroup because the attach callback 
will increment the counter and the fork callback will increment it again.

In attachment a source code to illustrate the example.

Shouldn't the ns_cgroup_clone be called after the cgroup_fork_callbacks 
in copy_process function ? So we don't call the fork callback for the 
first tasks and we keep the consistency ?

Thanks
  -- Daniel


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