Re: [PATCH 5/7] cgroups: Make freezer subsystem bindable

[Li Zefan <lizf@cn.fujitsu.com>]

Matt Helsley wrote:
> On Fri, Oct 22, 2010 at 04:11:41PM +0800, Li Zefan wrote:
>> To make it bindable, we need to thaw all processes when unbinding
>> the freezer subsystem from a cgroup hierarchy.
>>
>> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> 
> Based on experience using cgroups and questions we've fielded in the
> past on IRC I'd say users will really appreciate this.
> 
> We're planning to use the freezer in checkpoint/restart. Since
> checkpoint requires the tasks to remain frozen for the duration of
> the syscall we add a kernel-internal freezer subsystem interface
> which prevents the cgroup from thawing. So we'll need some way to
> "block" unbinding for that as well.
> 

How about, we allow unbinding only when all the cgroups' state is
THAWED ?

> Perhaps the bind op should be able to return an error when
> unbind == true? Although that raises the question of what to do if
> only one of multiple unbinds fails..
> 
> Alternately you could split the bind/unbind op function pointers
> and get rid of the boolean argument. Then just don't fill in the
> freezer's unbind op and refuse to unbind subsystems that lack
> the unbind op. That seems a bit cleaner for now at least.
> 

We can use bindable:1 along with a callback can_bind().

For some subsystems, we just set bindable to true. For freezer subsystem,
we set it to true and provide freezer_can_bind(), and return true only
if no cgroup's state is FROZEN.